UC-NRLF 


B   M    5ED   fihfl 


ASTRONOMY 


TEN  YEARS'  WORK 


OF    A     MOUNTAIN 


O  BS  E  R V ATO  R Y 


Mount  San  Antonio  from  Mount  Wilson. 


TEN  YEARS'  WORK 

OF  A 

MOUNTAIN  OBSERVATORY 


A  brief  account  of  the  Mount 
Wilson  Solar  Observatory  of  the 
Carnegie  Institution  of  Washington 

BY 

GEORGE  ELLERY  HALE 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
1915 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  235 


NOTE. 

The  eleven  departments  of  research  of  the  Carnegie  Insti- 
tution of  Washington,  of  which  the  Mount  Wilson  Solar 
Observatory  is  one,  are  located  in  various  places  selected 
because  of  their  suitability  for  the  several  purposes  in  view. 
Information  regarding  the  work  of  the  Institution  may  be 
obtained  on  application  to  the  Office  of  Administration, 
Washington  D.  C. 


ASTRONOMY  LIBRARY 


GIBSON  BROTHERS,  PRINTERS 
WASHINGTON,  D.  C. 


ASTRO  NO  to 
LIBRARY 


FIG.  i. — Mount  Wilson  from  Pasadena 


TEN  YEARS'  WORK  OF  A  MOUNTAIN 
OBSERVATORY 


Ten  years  have  elapsed  since  the  inception  of 
the  Mount  Wilson  Observatory,  and  the  time  is 
opportune  for  a  general  survey  of  the  work  of  the 
past  and  the  possibilities  of  the  future.  An  exten- 
sive equipment  of  buildings  and  instruments, 
involving  heavy  'initial 'cost  and  large  annual  ex- 
penditure for  operation  and  maintenance,  has  been 
provided  on  a  6,ooo-foot  mountain  summit  and  in 
the  valley  near  its  base.  Activities  of  many 
kinds,  including  the  preparation  of  new  plans  of 
research,  the  invention,  design,  and  construction 
of  instruments,  the  building  of  a  mountain  road, 

(3) 

M776894 


4 

the  transportation  of  many  hundreds  of  tons  of 
materials,  the  erection  of  brick,  steel,  and  concrete 
structures,  the  execution  of  an  observational  pro- 
gram, the  measurement  and  reduction  of  thou- 
sands of  photographs,  and  the  imitation  of  celes- 
tial phenomena  by  laboratory  experiments,  have 
taxed  the  capacity  of  a  large  staff  of  workers.  The 
equipment  is  now  so  nearly  complete  and  the  plan 
of  investigation  so  definitely  outlined  that  a  brief 
description  of  some  typical  methods  and  results 
may  be  of  service  to  the  visitor  and  to  the  general 
reader  interested  in  the  progress  of  astrophysical 
research. 

OLD    METHODS    AND    NEW. 

An  observatory,  like  any  other  laboratory  of 
research,  may  concentrate  its  attention  upon  either 
one  of  two  widely  different  objects:  the  accumu- 
lation of  great  stores  of  data  in  existing  depart- 
ments of  knowledge,  or  the  opening  up  and  explo- 
ration of  new  fields  of  investigation.  In  both  cases 
extensive  series  of  routine  observations  are  re- 
quired, but  the  point  of  view  and  the  mode  of 
attack  are  essentially  different.  In  known  fields, 
long  since  efficiently  occupied,  standard  methods 
of  observation  and  instruments  obtainable  from 
skilled  makers  are  available  for  use.  With  the  aid 
of  such  instruments  and  methods,  perhaps  modi- 
fied and  perfected  in  various  details,  observations 
of  great  precision  and  importance  can  be  obtained. 
Moreover,  by  the  preparation  of  a  suitable  scheme, 
well  exemplified  in  Kapteyn's  work  on  the  struc- 


5 

ture  of  the  universe,  these  observations  can  be 
made  to  serve,  not  merely  for  the  tabulation  of 
accurate  data,  but  for  the  solution  of  the  greatest 
problems  of  astronomy. 

Results  of  the  highest  importance  are  therefore 
within  the  reach  of  the  investigator  equipped  with 
standard  instruments.  His  studies  may  develop 
new  points  of  view  or  new  data  which  will  lead  into 
new  fields  of  research,  but  his  position  and  needs 
are  very  different  from  those  of  the  man  whose  re- 
searches force  him  to  leave  the  familiar  path. 
Discarding,  perhaps,  the  instruments  which  have 
proved  their  strength  and  weakness  by  many  years 
of  use,  he  replaces  them  with  others  possessing  new 
advantages  and  defects.  In  departing  from  ac- 
cepted standards  and  in  preparing  to  overcome 
difficulties,  the  initiator  of  new  methods  almost 
necessarily  becomes  an  instrument-maker,  and 
hence  a  machine-shop  may  be  his  first  requirement. 
He  can  not  afford  to  intrust  construction  to  instru- 
ment-makers thousands  of  miles  away,  with  whom 
he  is  unable  to  discuss  details  of  the  design,  neces- 
sarily subject  to  frequent  modification  in  the  light 
of  newly  acquired  ideas.  To  be  most  efficient,  he 
must  be  his  own  designer  and  builder,  ready  to 
take  immediate  advantage  of  those  new  points  of 
view  and  new  possibilities  of  attack  which  his 
investigations  are  certain  to  disclose. 

A  laboratory  or  observatory  like  that  of  Mount 
Wilson,  planned  for  the  exploration  of  unfamiliar 
fields,  can  thus  possess  no  fixed  and  standard 


equipment.  Its  mode  of  attack  and  its  means  of 
progress  must  grow  with  its  work  and  develop 
with  the  disclosure  of  new  and  unexpected  possi- 
bilities. 

ADVANTAGES    OF    A    MOUNTAIN    SITE. 

It  was  Newton  who  first  pointed  out  the  impor- 
tance of  making  astronomical  observations  from  a 
mountain  top:  "For  the  Air  through  which  we 
look  upon  the  Stars,  is  in  a  perpetual  Tremor;  as 
may  be  seen  by  the  tremulous  Motion  of  Shadows 
cast  from  high  Towers,  and  by  the  twinkling  of  the 
fix'd  stars.  .  .  .  The  only  remedy  is  a  most 
serene  and  quiet  Air,  such  as  may  perhaps  be 
found  on  the  tops  of  the  highest  Mountains  above 
the  grosser  Clouds."  (Opticks,  third  edition,  p.  98.) 

But  height  is  not  the  only  essential;  indeed,  very 
great  altitudes  are  to  be  avoided.  The  summit  of 
the  Rocky  Mountains  is  a  notoriously  bad  place 
for  astronomical  work,  because  of  the  unstable 
atmospheric  conditions  and  the  frequent  storms. 
Long  periods  of  unbroken  weather,  free  from  rain 
and  with  little  cloud,  are  associated  with  that 
tranquillity  and  steadiness  of  the  atmosphere  which 
Newton  so  much  desired.  These  are  to  be  found 
on  the  mountains  of  the  Sierra  Madre  range,  in 
the  semi-tropical  climate  of  Southern  California. 
Mount  Wilson,  selected  by  Hussey  after  many 
tests  of  other  elevated  points  in  the  northern  and 
southern  hemispheres,  was  accordingly  chosen  as 
the  observation  station.  Rising  abruptly  from 


the  San  Gabriel  Valley  to  a  height  of  nearly  6,000 
feet,  and  lying  some  30  miles  from  the  Pacific 
Ocean,  it  offered  more  advantages  than  any  other 
site  known.  One  of  these  was  the  possibility  of 
establishing  the  shops,  laboratories,  and  offices  in 
the  city  of  Pasadena,  within  easy  reach  of  large 
foundries,  supply  houses,  sources  of  electric  light 
and  power,  and  other  facilities  demanded  by  the 
nature  of  the  work.  Throughout  the  dry  season, 


FIG.  2. — Mount  Wilson  from  Mount  Harvard. 

day  after  day  is  clear  and  tranquil  and  the  wind 
velocity  is  remarkably  low.  The  broad  and  diver- 
sified mountain  summit,  protected  from  the  sun's 
heat  by  spruces,  pines,  and  undergrowth,  affords 
numerous  locations  perfectly  adapted  for  the  vari- 
ous instruments;  the  water-supply  is  abundant, 
and  the  completion  of  the  mountain  road  makes 
the  distance  to  the  Pasadena  office  only  16  miles, 
covered  in  i\  hours  (ascending)  by  auto-stage. 


LIFE    HISTORY    OF    THE    STARS. 

But  the  scheme  of  research  is  the  prime  con- 
sideration. Let  us  suppose,  as  in  the  case  of  the 
Mount  Wilson  Observatory,  that  the  chief  object 
is  to  contribute,  in  the  highest  degree  possible,  to 
the  solution  of  the  problem  of  stellar  evolution. 
What  was  the  origin  of  this  earth  on  which  we  live? 
We  know  that  it  is  a  member  of  a  solar  system,  one 
of  several  planets  moving  harmoniously  about  a 
great  central  sun,  on  which  they  depend  for  light 


FIG.  3. — Saturn. 

and  heat.  But  how  was  the  earth  formed? 
Through  what  successive  stages  did  it  pass  in  its 
early  life?  How  were  its  constituent  parts  sepa- 
rated from  that  great  vaporous  mass  which,  as 
most  astronomers  believe,  once  united  the  planets 
and  the  sun?  By  what  process,  extending  over 
millions  of  years,  have  the  intensely  hot  solar  gases 
condensed  toward  a  center,  leaving  behind  those 
rotating  and  revolving  spheres,  the  planets  and 
their  satellites  ?  Or  must  this  Laplacian  view  give 


way  to  the  radically  different  conceptions  of  the 
planetesimal  hypothesis?  What  is  the  nature  of 
the  central  sun,  on  which  our  lives  depend  ?  What 
is  its  relationship  to  other  stars,  and  what  part 
does  it  play  in  the  universe  ?  How  is  this  universe 
organized,  what  bodies  does  it  comprise,  what  is 
its  structure,  and  what  are  its  bounds?  If  the 
processes  of  creation  and  evolution  are  still  at  work 
within  it,  may  we  not  expect  to  find  existing  exam- 


FIG.  4. — Nebula  N.  G.  C.   7217. 

pies  of  the  various  stages  through  which  our  own 
solar  system  has  passed?  And  may  we  not  hope, 
by  learning  the  relationship  of  stars  in  moving 
groups,  and  by  tracing  these  groups  back  to  their 
former  positions  in  space,  to  reconstruct  a  picture 
of  the  universe  as  it  was  long  before  the  solar  sys- 
tem had  taken  form? 

A  large  project,  it  may  be  said,  too  ambitious  for 
the  ideal  of  any  institution.     But  while  no  single 


10 

observatory  may  hope  to  cover  the  ground  com- 
pletely, it  may  reasonably  expect  to  contribute 
toward  the  solution  of  the  problem  and  thus  to  aid 
in  attaining  a  clearer  comprehension  of  that  great 
process  of  evolution  which  has  produced  the  earth 
and  its  inhabitants.  It  is  evident,  from  the  nature 
of  the  case,  that  the  plan  of  attack  must  be  broad 
and  elastic,  utilizing  a  variety  of  powerful  methods 
toward  a  common  end.  But  in  the  presence  of 
innumerable  interesting  objects  for  study,  there  is 
danger  of  a  scattering  of  effort  and  a  mere  multi- 
plication of  observations.  Every  possible  astro- 
nomical observation  might  have  a  bearing  on  our 
problem  and  thus  seem  to  justify  its  making;  but 
unless  it  were  made  as  an  element  in  some  general 
plan,  an  indefinite  amount  of  energy  might  be 
spent  without  avail.  A  common  scheme  should 
tie  together  many  diverse  investigations,  multi- 
plying the  intrinsic  importance  of  each  because  of 
its  bearing  on  all  the  others. 

THE  SUN. 

Let  us  begin  with  the  sun.  Its  prime  interest 
and  importance,  as  the  source  of  the  light  and  heat 
on  which  we  all  depend,  would  be  sufficient  reason 
for  its  special  consideration.  But  equally  signifi- 
cant is  the  fact  that  the  sun  is  the  only  one  of  all  the 
stars  which  lies  near  enough  to  the  earth  to  be 
studied  in  detail;  each  of  the  others  is  reduced  by 
distance  to  an  infinitesimal  point  of  light,  which  the 
most  powerful  of  telescopes  can  not  magnify  into  an 


II 

appreciable  disk.  We  may  safely  infer,  from  many 
observations  of  recent  years,  that  thousands  of  the 
stars  are  almost  identical  in  character  with  the 
sun,  though  some  are  much  larger  or  smaller  and 
some  are  in  earlier  or  later  stages  of  development. 
But  if  we  wish  to  know  what  a  star  really  is  we 


FIG.  5. — Direct  photograph  of  the  Sun. 

must  approach  it  closely,  and  this  is  possible  only 
in  the  case  of  the  sun.  Indeed,  because  the  sun 
was  regarded  as  so  important,  offering  so  many 
opportunities  to  increase  our  knowledge  of  its 
nature,  the  observatory  was  conceived  primarily 


12 

for  solar  research.  But  the  necessity  for  seeking, 
among  the  stars  and  nebulae,  for  evidence  as  to  the 
past  and  future  stages  of  solar  and  stellar  life, 
rendered  a  broadening  of  scope  advisable  from 
the  outset.  Much  attention  is  therefore  devoted 
to  the  sun  as  the  chief  among  the  stars,  but  the 
essential  means  of  attacking  the  more  distant 
objects  of  the  universe  have  also  been  provided. 

AUXILIARIES    OF    THE    TELESCOPE. 

Ten  years  ago  the  possibilities  of  the  spectro- 
heliograph,  as  a  means  of  increasing  our  knowledge 
of  the  invisible  atmosphere  of  the  sun,  had  become 
apparent.  This  instrument,  which  was  clearly 


FIG.  6. — The  Kenwood  Spectroheliograph. 

susceptible  of  further  improvement  and  develop- 
ment, was  accordingly  chosen  as  one  of  our  chief 
auxiliaries  for  the  study  of  the  sun.  Of  still  wider 
range  of  application  was  the  solar  spectroscope, 
previously  used  almost  exclusively  as  a  visual 
instrument  of  small  dimensions  attached  to  a 
moving  telescope  tube.  Rowland  had  invented 


13 

the  concave  grating,  and  used  it  for  his  epoch- 
making  photographic  studies  of  laboratory  spectra 
and  the  spectrum  of  sunlight.  But  his  solar  image 
was  small  and  unsteady  and  there  remained  a  most 
promising  opportunity  to  apply  a  powerful  photo- 
graphic spectroscope  to  the  investigation  of  sun- 
spots,  the  chromosphere,  and  other  details  of  a 
large  solar  image.  Solar  spectroscopy  was  far 
behind  laboratory  spectroscopy  and  new  types  of 


FIG.  7. — The  Snow  Telescope. 

instruments  were  clearly  demanded.  It  was  evi- 
dent that  for  efficient  use  in  photography  the 
spectroscope  of  the  day  must  be  greatly  lengthened, 
thus  making  it  too  long  to  serve  as  an  attachment 
to  a  moving  telescope.  Accordingly  it  became 
necessary  to  develop  a  suitable  form  of  fixed  tele- 
scope, capable  of  forming  a  large  and  sharply 
defined  image  of  the  sun  on  the  slit  of  a  long  fixed 
spectrograph. 


THE  SNOW  TELESCOPE. 

A  step  in  this  direction  was  the  horizontal  tele- 
scope, which  the  late  Miss  Snow  had  presented  to 
the  Yerkes  Observatory  through  the  kind  interest 
and  assistance  of  Dr.  George  S.  Isham.  It*con- 
sists  of  a  coelostat,  with  a  plane  mirror  30  inches 
in  diameter,  rotated^by  clockwork  at  such  a  rate 


FIG.  8. — Ccelostat  and  second  mirror  of  Snow  Telescope. 

as  to  keep  the  beam  of  sunlight,  reflected  from  its 
silvered  (front)  surface,  in  a  fixed  position  on  a 
second  plane  mirror  standing  above  and  south 
of  it.  From  this  mirror  the  beam  is  reflected 
nearly  horizontally  to  a  point  100  feet  north,  where 
it  falls  on  a  24-inch  concave  mirror  of  60  feet  focal 
length,  which  forms  a  solar  image  about  6^2  inches 


15 

in  diameter  on  the  slit  of  the  spectrograph  or 
spectroheliograph. 

Loaned  by  the  University  of  Chicago,  and  set  up 
on  Mount  Wilson  at  a  time  when  the  Solar  Obser- 
vatory was  at  work  as  an  expedition  from  the 
Yerkes  Observatory,  the  Snow  telescope  was  found 
to  have  advantages  and  defects  characteristic  of  a 
new  instrument.  Currents  of  warm  air,  rising 
from  the  hot  soil  of  the  mountain  summit  and 
carried  across  the  entering  beam  of  light,  decreased 
the  sharpness  of  the  image  during  the  hotter  hours 
of  the  day.  The  sun's  direct  rays  warped  the 
telescope  mirrors,  changing  the  focus  and  blurring 
the  details  of  the  image  after  a  few  minutes  of 
exposure.  But  the  worst  of  these  difficulties  were 
soon  overcome  by  observing  in  the  early  morning 
or  late  afternoon,  shielding  the  mirrors  from  the 
sun  between  exposures  and  cooling  them  with 
blasts  from  electric  fans.  Thus  controlled,  the 
Snow  telescope  yielded  excellent  photographs  of 
the  solar  atmosphere  and  justified  the  hopes  we 
had  entertained  of  its  performance. 

WHAT    IS    A    SUN-SPOT? 

Sun-spots,  though  known  and  studied  for  300 
years,  offered  most  promising  opportunities  for 
research.  Evidently  there  was  much  to  be  learned 
from  an  investigation  of  their  spectra,  which  had 
never  been  attempted  with  adequate  instrumental 
means.  To  produce  these  spectra,  the  light  of  a 
sun-spot  was  passed  through  a  narrow  slit,  and 


i6 

thence  to  a  lens  of  18  feet  focal  length,  which 
rendered  the  rays  parallel;  they  then  met  a  grating 
of  polished  metal,  ruled  with  some  15,000  lines  to 
the  inch;  this  analyzed  the  composite  light  into 
its  constituent  parts  and  returned  the  rays  through 
the  lens,  which  formed  an  image  of  the  long  spec- 
trum band  on  a  photographic  plate  below  the  slit. 
With  this  spectrograph,  constructed  in  our  shop 
in  Pasadena  and  mounted  for  use  with  the  Snow 


FIG.  9. —Direct  photograph  of  Sun-spot. 

telescope,  it  soon  became  an  easy  matter  to  photo- 
graph sun-spot  spectra.  The  curious  widened 
lines,  the  much  debated  bands,  and  the  strength- 
ened and  weakened  lines  were  thus  accurately 
recorded  for  study.  What  is  the  cause  of  these 
peculiarities?  The  first  step  was  to  test  by  labor- 
atory experiments  the  hypothesis  that  some  of 
them  are  due  to  reduced  temperature  of  the  spot 
vapors.  In  studying  such  phenomena  the  spec- 
troscopist  is  in  a  position  much  like  that  of  an 


archeologist  endeavoring  to  translate  an  unknown 
language.  A  bilingual  inscription,  containing  an 
expression  of  the  same  fact  in  both  celestial  and 
terrestrial  characters,  is  what  he  requires,  and  this 
a  suitably  equipped  physical  laboratory  is  often 
capable  of  supplying. 

In  the  solar  spectrum  we  can  photograph  about 
20,000  lines,  distributed  irregularly  from  the  red 
to  the  violet,  and  throughout  the  invisible  regions 
beyond.  Perhaps  some  of  these  are  due  to  iron. 


FIG.    10. — Sun-spot   Spectrum,     a   Solar    and    b   Spot  Spectrum 
widened;  c  from  original  negative,  Spot  Spectrum  in  middle. 

To  settle  this  it  is  only  necessary  to  vaporize  some 
iron  between  the  poles  of  an  electric  arc  and  photo- 
graph its  spectrum  beside  that  of  the  sun  (Fig.  n). 
Some  2,000  solar  lines  are  found  to  coincide  in 
position  with  lines  of  iron.  As  these  lines  are 
given  only  by  iron,  we  may  conclude  at  once  that 
this  element  exists  in  the  solar  atmosphere. 

So  much  for  the  chemical  identification  of  lines. 
We  may  next  interpret  their  peculiarities.  In  the 
case  of  sun-spots  we  suspected  that  certain  changes 


i8 


in  the  relative  intensities  of  the  lines  were  due  to 
a  reduced  temperature  of  the  spot  vapors.  To  test 
this,  the  spectrum  of  iron  vapor  in  an  electric  arc 
was  photographed  at  different  temperatures. 
Some  of  the  lines  were  found  to  strengthen,  others 
to  weaken  relatively,  as  the  temperature  was 


FIG.  ii. — Iron  (above)  and  Solar  Spectrum  (below). 

reduced.  When  compared  with  the  iron  lines  in 
sun-spots  the  changes  were  seen  to  be  of  the  same 
kind.  The  same  test,  applied  to  the  vapors  of 
chromium,  nickel,  manganese,  titanium,  and  other 
metallic  elements,  previously  identified  in  spots, 


FIG.  12. — Effect  of  temperature  on  Spectrum  of  Vanadium:  a  In  Carbon 
Arc;  b,  c,  and  d  in  Electric  Furnace  at  temperatures  of  2600°, 
2350°,  and  2150°  C.,  respectively. 


19 

gave  the  same  result.  It  thus  became  clear  that 
sun-spots  actually  are  regions  of  reduced  tempera- 
ture in  the  solar  atmosphere. 

The  next  step  bore  out  this  conclusion.  If  the 
solar  vapors  are  cooler  in  spots  than  in  the  general 
atmosphere  of  the  sun,  then  it  may  be  possible  for 
some  of  them  to  unite  chemically.  Thousands  of 
faint  lines  in  the  spot  spectra  were  measured  and 
identified  as  band  lines  dueto  chemical  compounds. 
Fowler,  who  had  also  worked  with  success  on  the 
strengthened  and  weakened  lines,  found  magne- 
sium hydride.  Titanium  oxide  and  calcium  hy- 
dride were  identified  in  our  laboratory.-  Thus  we 
began  to  form  a  new  picture  of  these  regions  of  the 
solar  atmosphere  and  to  recognize  the  chemical 
changes  at  work  in  the  spot  vapors. 

SOLAR    METEOROLOGY. 

Meanwhile  systematic  work  was  in  progress 
with  the  spectroheliograph,  which  gives  images  of 
the  sun  in  monochromatic  light,  showing  the  dis- 
tribution of  some  one  vapor  in  its  atmosphere. 
In  the  favorable  California  climate  it  is  possible 
to  photograph  the  sun  on  about  300  days  of  the 
year  (in  one  season  on  113  successive  days). 
Every  clear  morning,  and  frequently  in  the  after- 
noon, the  instrument  was  at  work,  making  pictures 
of  the  great  gaseous  clouds  in  the  solar  atmosphere. 
These  were  first  observed  many  years  ago  as  solar 
prominences,  rising  high  above  the  sun's  limb  at 
total  eclipses,  when  the  bright  light  of  the  disk  was 


20 

cut  off  by  the  moon.  The  spectroheliograph  not 
only  permits  the  prominences  to  be  photographed 
on  any  clear  day,  but  discloses  extensive  clouds  of 
calcium,  hydrogen,  iron,  and  other  vapors,  which 
do  not  rise  high  enough  to  be  observed  in  elevation 
at  the  limb,  but  are  recorded  (as  flocculi)  in  pro- 
jection against  the  bright  disk.  To  the  eye  at  the 
telescope,  or  in  direct  photographs  of  the  ordinary 


FIG.    13. — The  Chromosphere  photographed   without  an   IJclipse. 

kind,  these  flocculi  are  wholly  invisible.  The  spec- 
troheliograph brings  them  to  view  by  excluding 
from  the  photographic  plate  all  light  except  that 
due  to  calcium  or  hydrogen,  as  the  case  may  be. 
The  measurement  of  these  plates  with  the  helio- 
micrometer  (Fig.  55),  an  instrument  devised  and 
constructed  in  our  instrument-shop,  gave  directly 
the  latitudes  and  longitudes  of  the  flocculi,  without 


21 

the  extensive  computations  required  when  the  ordi- 
nary type  of  measuring  machine  is  used.  Their 
change  of  position. from  day  to  day  yielded  a  new 
determination  of  the  law  of  the  solar  rotation, 
which  was  found  to  differ  at  the  calcium  and 
hydrogen  levels.  At  the  lower  level  of  the  calcium 
flocculi  the  period  of  rotation  at  the  sun's  equator 
is  24.8  days,  increasing  gradually  to  26.8  days  at 
45°  latitude.  In  other  words,  the  gaseous  sun 


KIG.  14. — Solar  Prominence  80,000  miles  high. 

does  not  rotate  like  the  solid  earth,  on  which  points 
in  all  latitudes  complete  a  rotation  in  24  hours. 
It  turns  more  and  more  slowly  as  the  poles  are 
approached,  points  in  high  latitudes  lagging  behind 
those  nearer  the  equator.  If  this  could  happen  on 
the  earth,  Jacksonville,  which  is  almost  due  south 
of  Cleveland,  would  be  far  to  the  east  of  it  24 
hours  hence.  In  the  higher  levels  of  the  solar 


22 

atmosphere,  where  the  hydrogen  flocculi  float,  the 
period  of  rotation  for  any  latitude  is  less  than  for 
the  levels  below,  but  the  difference  in  rotation 
time  between  pole  and  equator  is  less  marked  than 
in  the  lower  atmosphere. 


FIG.  15. — a,  Direct  photograph  of  Sun,  August  31,  1906;  b.  Calcium 
(Hz)  Flocculi  at  same  hour. 


SOLAR  AND  STELLAR  SPECTROSCOPY. 

Since  the  flocculi  are  constantly  changing  in 
form,  they  are  not  very  satisfactory  objects  for 
rotation  measurements.  Much  more  accurate 
results  can  be  obtained  by  measuring,  with  a 
powerful  spectrograph,  the  velocity  of  approach 
and  recession  of  the  east  and  west  edges  of  the  sun. 
The  east  edge  is  moving  toward  the  earth  on 
account  of  the  sun's  rotation;  this  causes  a  dis- 
placement of  the  spectrum  lines  toward  the  violet 
(Fig.  1 6) .  At  the  west  edge,  which  is  moving  away, 
the  lines  are  equally  displaced  toward  the  red.  The 
double  displacement,  measured  at  different  lati- 


23 

tudes,  gives  the  velocity  of  approach  and  recession 
in  kilometers  per  second.  An  investigation  of  this 
kind  threw  much  new  light  on  the  peculiar  law  of 
the  solar  rotation,  giving  with  high  precision  the 
rotation  period  at  different  levels  and  the  change 
in  its  value  from  equator  to  pole. 

The  Snow  telescope  thus  proved  its  usefulness 
for  a  wide  variety  of  observations,  most  of  which 
we  could  not  have  made  with  moving  telescopes 
of  the  standard  type.  In  addition  to  the  work 
already  mentioned,  the  1 8-foot  spectrograph 


FIG.  16. — Spectra  of  east  (a)  and  west  (ft)  edges  of  Sun,  showing  dis- 
placement caused  by  solar  rotation. 

yielded  excellent  photographs  of  spectra  of  various 
parts  of  the  solar  disk,  revealing  numerous  pecu- 
liarities in  the  spectrum  near  the  edge  of  the  sun. 
Although  not  designed  for  stellar  work,  the  Snow 
telescope  also  permitted  photographs  of  the  spec- 
trum of  Arcturus  to  be  taken  with  a  powerful 
grating  spectrograph.  When  compared  with  the 
spectra  of  sun-spots,  the  relative  intensities  of  the 
lines  were  found  to  be  similar,  indicating  that 
Arcturus  is  cooler  than  the  sun,  a  fact  of  impor- 
tance in  its  bearing  on  the  question  of  stellar 
evolution. 


24 
THE    6O-FOOT   TOWER   TELESCOPE. 

But  the  Snow  telescope  was  not  free  from  limi- 
tations. During  long  exposures  its  mirrors  were 
seriously  distorted  by  the  sun's  heat  and  the  effect 
of  heated  air  from  the  earth  was  plainly  shown  by 
a  blurring  of  the  solar  image.  To  obviate  or  reduce 


FIG.  17. — 6o-foot  Tower  Telescope. 

these  difficulties  the  vertical  or  tower  telescope 
was  devised,  and  constructed  in  an  inexpensive 
form.  After  reflection  from  two  plane  mirrors  at 
the  summit,  the  sun's  rays  pass  through  a  1 2-inch 
objective  of  60  feet  focal  length,  which  forms  an 
image  of  the  sun  on  the  slit  of  the  spectrograph  in 
the  observing-room  at  the  foot  of  the  tower.  The 


FIG.  1 8. — Hydrogen  Flocculi  surrounding  Sun-spots,  showing 
right-  and  left-handed  Vortices,  September  9,  1908. 

mirrors,  much  thicker  than  those  of  the  Snow 
telescope,  are  but  little  affected  by  the  sun's  heat. 
Elevated  60  feet  in  the  air,  they  also  escape  some 
of  the  warm  currents  rising  from  the  hot  soil. 
The  results  are  a  decided  improvement  in  the 
sharpness  of  the  image  and  a  prolongation  of  the 
period  during  which  good  observations  are  possible. 
Another  advantage,  quite  as  important,  follows 
from  the  use  of  an  underground  chamber  to  con- 


26 


tain  the  spectrograph,  now  increased  in  length 
from  1 8  to  30  feet.  The  gain  resulting  from  its 
greater  stability  and  from  the  constancy  of  tem- 
perature of  the  grating  at  the  bottom  of  the  well 
was  plainly  apparent  in  the  new  photographs  of 
spot  spectra,  which  brought  out  details  previously 
unrecognized. 

SOLAR    VORTICES    AND    MAGNETIC    FIELDS. 

The  development  of  the  spectroheliograph  had 
also  advanced  another  step.     The  dark  hydrogen 


June  2 


June  3 
4h58mP.M. 


June  3 
5t>i4">  P.M. 


June  3 
5h  22m 


FIG.    19. — Hydrogen   Flocculus  sucked   into  Sun-spot,   June   3,    1908. 


flocculi,  first  photographed  at  the  Yerkes  Observa- 
tory in  1903,  had  hitherto  been  recorded  only  with 
the  blue  hydrogen  lines.  In  1908  the  new  red- 
sensitive  plates  of  Wallace,  applied  to  the  pho- 
tography of  the  sun's  disk  with  the  red  (Ha)  line  of 
hydrogen,  gave  results  of  great  interest.  In  the 


27 

higher  part  of  the  hydrogen  atmosphere,  thus 
revealed  in  projection  against  the  disk,  immense 
vortices  were  found  surrounding  sun-spots  (Fig.  1 8) . 
This  led  to  the  hypothesis  that  a  sun-spot  is  a  solar 
storm,  resembling  a  terrestrial  tornado,  in  which 
the  hot  vapors,  whirling  at  high  velocity,  are 
cooled  by  expansion,  thus  accounting  for  the 


FIG.  20. — jo-foot  Spectrograph,  with  Polarizing  Apparatus  above  Slit. 

observed  intensity  changes  of  the  spectrum  lines 
and  the  presence  of  chemical  compounds. 

But  the  observed  widening  of  many  spot  lines 
and  the  doubling  or  trebling  of  some  of  them  re- 
mained inexplicable  until  the  vortex  hypothesis 
suggested  an  explanation.  Thomson  and  others 
had  shown  that  electrons  are  emitted  by  hot  bodies; 
hence  they  must  be  present  in  great  numbers  in 


28 

the  sun.  If  positive  or  negative  electrons  were 
caught  and  whirled  in  a  vortex  they  would  produce 
a  magnetic  field,  such  as  we  obtain  by  passing  an 
electric  current  through  a  coil  of  wire.  Zeeman 
had  discovered  in  1896  that  the  lines  in  the  spec- 
trum of  a  luminous  vapor  in  a  magnetic  field  are 
widened  or  (if  the  field  is  strong  enough)  split  into 
several  components  (Fig.  21).  Moreover,  the  light 


FIG.  21. — Effect  of  Magnetic  Field  upon  Lines  of  Iron  Spectrum.  In 
a  the  middle  component  and  in  b  the  outer  components  are  cut 
out  by  a  Nicol  Prism;  c,  Spectrum  without  Magnetic  Field. 

of  these  components  is  polarized  in  so  characteristic 
a  way  that  there  can  be  no  uncertainty  in  identi- 
fying the  effect.  Could  this  be  the  condition  of 
things  in  sun-spots? 

The  3o-foot  spectrograph  of  the  tower  telescope 
permitted  the  test  to  be  made  at  once.  The 
characteristic  polarization  phenomena  appeared 
and  one  by  one  all  of  the  distinctive  peculiarities 
of  the  Zeeman  effect  were  made  out.  Thus  direct 


evidence,  open  to  only  one  interpretation,  proved 
the  existence  of  magnetic  fields  in  sun-spots,  and 
strengthened  the  view  that  these  are  caused  by 
electric  vortices.  This  conclusion,  in  common 
with  many  others  regarding  the  nature  of  sun- 
spots,  could  not  have  been  obtained  without  the 
aid  of  the  physical  laboratory. 

A  B 


FIG.  22. — Effect  of  Nicol  and  Compound  Quarter-wave  Plate  upon  (.4) 
Lines  of  Spark  in  Magnetic  Field  and  (fl)  Solar  Line  in  Spectrum 
of  Sun-spot,  a  Red  Components;  b  Violet  Components;  middle 
line  of  Triplet  shows  in  B  but  not  in  A. 

Let  us  see  how  the  effect  of  magnetism  on  light 
is  studied.  We  place  our  iron  arc  or  spark  between 
the  poles  of  a  powerful  magnet  (Fig.  29)  and  pho- 
tograph its  spectrum.  The  lines  behave  in  the  most 
diverse  way,  some  splitting  into  triplets,  others  into 
quadruplets,  quintuplets,  sextuplets,  etc.  One 
chromium  line  is  resolved  by  the  magnet  into  21 


30 

components.  If  a  magnetic  field  is  really  at  work  in 
sun-spots  we  should  anticipate  a  close  correspond- 
ence between  the  behavior  of  each  solar  line  and 
its  laboratory  equivalent.  And  this  is  exactly 
what  we  find  (Fig.  22).  Furthermore,  the  distance 
between  the  components  of  a  line  is  directly  pro- 
portional to  the  strength  of  the  magnetic  field. 
Thus,  by  determining  the  separation  correspond- 
ing to  a  magnetic  field  whose  strength  can  be 
measured  in  the  laboratory,  we  may  easily  derive 
the  strength  of  the  field  in  sun-spots. 


FIG.  23. — Waterspout  off  the  Coast  of  Sicily. 
SUN-SPOTS    AND    FLOCCULI. 

We  can  not  enter  here  into  the  various  appli- 
cations of  this  conclusion  to  the  explanation  of 
solar  phenomena.  If  we  could  see  a  single  sun- 
spot  from  a  point  beneath  the  solar  surface,  it 
would  probably  resemble  a  terrestrial  water-spout 
or  tornado,  though  its  cross-section,  instead  of 
being  a  few  hundred  feet,  would  be  hundreds  of 
miles.  The  strength  of  the  magnetic  field  pro- 
duced, which  is  measured  by  the  degree  of  separa- 


tion  of  the  triple  lines,  increases  with  the  diameter 
of  the  spot.  The  field  is  strongest  near  the  center 
of  the  spot,  where  the  lines  of  the  triplet  are  most 
widely  separated,  and  decreases  to  very  low  in- 
tensity at  points  just  outside  the  edge  of  the 
penumbra.  The  spectrograph,  when  equipped 
with  suitable  polarizing  apparatus,  serves  as  an 
extraordinarily  delicate  means  of  measuring  these 
fields,  which  can  be  observed  in  regions  where 
they  are  not  much  more  intense  than  the  magnetic 
field  of  the  earth.  In  this  way  it  became  possible, 


FIG.  24. — Lines  of  Force  about  +  and  —  Poles  of  a  Magnet. 

as  described  below  (p.  43),  to  detect  the  compara- 
tively weak  magnetic  field  of  the  entire  sun. 

It  has  long  been  known  that  sun-spots  usually 
occur  in  pairs,  and  our  study  of  the  Zeeman  effect 
indicates  that  the  two  principal  spots  in  such  a 
group  are  almost  invariably  of  opposite  polarity. 
The  natural  inference  is  that  we  are  here  dealing 
with  a  semi-circular  vortex  (like  half  a  smoke  ring) 
the  two  ends  of  which,  coming  to  the  sun's  surface 


32 

from  below,  appear  to  whirl  in  opposite  direc- 
tions. But  this  hypothesis  is  still  under  investi- 
gation. Stormer  of  Christiania  has  developed  for 
us  the  mathematical  theory  of  the  motions  in  the 
solar  atmosphere  of  vapors  within  the  influence  of 
such  magnetic  fields,  with  results  of  great  interest. 


FIG.  25.— Two  Bipolar  Sun-spot  Groups. 

The  illustration  (Fig.  25)  shows  that  the  hydrogen 
flocculi  about  a  bipolar  spot-group  resemble  the 
lines  of  force  between  two  magnets  of  opposite 
polarity  (Fig.  24).  But  similar  structure  might  be 
produced  by  the  direct  hydrodynamical  influence 
of  the  spot  vortices  upon  the  solar  atmosphere 


.33 

above  them,  and  it  is  still  a  question  whether  this 
or  the  electromagnetic  influence  is  predominant. 
The  existence  of  electric  vortices  in  sun-spots  is 
indirectly  shown  by  the  presence  of  the  magnetic 
fields,  which  presumably  could  not  be  produced  in 
the  sun  by  any  other  means.  But  direct-proof  of 
the  existence  of  these  vortices  was  subsequently 


FIG.  26. — Inflow  at  High  Levels  and  Outflow  at  Low  Levels  above  Spots. 

found  by  Evershed,  of  Kodaikanal,  who  measured 
the  outflow  of  gases  close  to  the  solar  surface  and 
their  inflow  at  higher  levels.  This  work  has  been 
repeated  and  extended  on  Mount  Wilson  with 
highly  significant  results,  affording  not  only  an 
accurate  picture  of  the  conditions  existing  above 
a  sun-spot,  but  also  the  means  of  assigning  to  each 
line  of  the  solar  spectrum  a  definite  level  in  the 
sun's  atmosphere. 


34 


WORK  OF  THE  PASADENA  LABORATORY. 

The  value  of  laboratory  work  in  the  interpreta- 
tion of  solar  phenomena  has  already  been  illus- 
trated. Magnetic  fields  are  detected  by  the  split- 
ting and  polarization  of  spectrum  lines,  differences 
of  temperature  by  changes  in  their  relative  inten- 
sities, differences  of  pressure  by  shifts  in  their 
positions,  etc.  By  producing  such  effects  artifi- 


FIG.  27. — Pasadena  Laboratory. 

cially,  with  the  aid  of  powerful  electric  furnaces, 
pressure  pumps,  and  other  physical  instruments, 
we  can  imitate  a  great  variety  of  celestial  phe- 
nomena and  interpret  complex  and  obscure  pecu- 
liarities. It  is  thus  plainly  apparent  that  a  phys- 
ical laboratory  is  a  necessary  adjunct  of  an  astro- 
physical  observatory.  Our  experience  has  shown 


35 

that  the  literature  of  spectroscopy  almost  never 
contains  the  information  we  require.  The  only 
way  to  obtain  sufficient  data  is  to  produce  them 
ourselves,  under  conditions  within  our  own  control 
and  adapted  to  meet  the  manifold  requirements 
of  the  observed  astronomical  phenomena.  More- 
over, the  performace  of  a  few  experiments  never 
suffices.  It  becomes  necessary,  not  only  to  imi- 
tate an  effect  for  a  given  line,  or  for  a  limited  region 
of  the  spectrum,  but  to  extend  the  observations 
over  the  whole  range  of  spectrum  available. 

It  is  easy  to  see  that  heavy  tasks  devolve  upon 
our  laboratory  staff,  both  in  observing  and  in  the 
extensive  work  of  measurement  and  reduction. 
It  is  a  comparatively  simple  matter  to  show  that  a 
change  of  furnace  temperature  will  modify  the 
relative  intensities  of  certain  lines;  but  to  measure 
the  changes  for  the  thousands  of  lines  of  iron, 
chromium,  nickel,  vanadium,  and  many  other 
elements  recognized  in  celestial  objects  is  a  task 
requiring  years  of  continuous  work.  So  with  all 
of  the  other  effects  of  pressure,  magnetic  field, 
change  of  potential  of  the  electric  discharge,  etc. 
It  is  evident  why  the  simple  beginnings  of  this 
laboratory  on  Mount  Wilson  have  led  to  larger 
developments  in  Pasadena,  where  heavy  electric 
currents  and  other  facilities  are  available. 

The  intrinsic  value  of  the  laboratory  results,  as 
distinguished  from  their  usefulness  for  the  inter- 
pretation of  astronomical  observations,  should  not 
be  overlooked.  Each  investigation  is  equally 


applicable  to  the  interpretation  of  fundamental 
problems  of  physics,  particularly  those  concerned 
with  the  nature  of  radiation.  The  changes  of 
intensity  of  spectrum  lines  produced  by  raising 
and  lowering  the  temperature  of  the  electric  fur- 
nace help  to  indicate  how  the  shock  of  molecular 
collisions  may  influence  the  motions  of  the  elec- 
trons within  the  atom.  The  phenomena  of  the 
"tube  arc"  throw  new  light  on  radiation  processes 
hitherto  associated  mainly  with  high  electric  po- 


FIG.  28. — Electric  Furnace. 

tentials.  The  complex  phenomena  of  the  Zeeman 
effect  (as  revealed  in  a  comparative  study,  with 
powerful  spectrographs  and  an  intense  magnetic 
field,  of  the  lines  of  a  long  list  of  elements)  furnish 
material  available  for  wide  generalizations,  impor- 
tant in  their  bearing  on  theories  of  radiation  and 
atomic  structure.  Thus  the  maintenance  of  this 
laboratory  would  be  highly  advantageous  from 
the  standpoint  of  the  physicist,  even  if  it  had  no 


37 

connection  with  the  Observatory.  The  doubled 
efficiency  resulting  from  the  combination  of  the 
two  is  of  the  same  character  as  that  which  follows 
from  the  joint  prosecution  of  the  solar  and  stellar 
work. 

PRESSURES    AND    MOTIONS    IN    THE 
SOLAR    ATMOSPHERE. 

I  have  mentioned  the  displacement  of  lines  by 
pressure  as  a  laboratory  problem.  The  applica- 
tion of  the  results  to  the  interpretation  of  line 


FIG.  29.— Magnet  for  Zeeman  Kffect. 

displacements  observed  in  various  parts  of  the 
solar  atmosphere  has  yielded  much  new  informa- 
tion. If  the  vapors  in  question  are  moving  toward 
or  away  from  the  observer,  displacements  due  to 
motion  must  be  distinguished  from  those  caused 
by  pressure;  this  can  be  done  with  certainty  by 


38 

using  the  laboratory  data,  some  lines  being  shifted 
by  pressure  to  the  red  and  others  to  the  violet.  It 
is  found  that  some  of  the  solar  gases  are  rising, 
while  others  are  falling.  The  pressure  at  different 
heights  is  then  determined,  and  found  to  range 
from  about  an  atmosphere  close  to  the  solar  surface 
to  exceedingly  low  values,  such  as  we  know  in 
vacuum  tubes,  at  elevations  of  several  thousand 
miles.  The  delicacy  of  this  method  is  illustrated 
by  the  fact  that  the  spectrographs  on  Mount 
Wilson  and  in  Pasadena  show  a  distinct  difference 
in  the  position  of  certain  lines  in  the  electric-arc 
spectrum,  caused  by  the  difference  in  atmospheric 
pressure  between  mountain  and  valley.  Thus 
we  are  enabled  to  sound  the  solar  atmosphere 
through  all  its  depths  and  to  learn  of  its  phenom- 
ena at  different  levels.  The  same  method,  when 
applied  to  stars,  has  given  us  a  preliminary  deter- 
mination of  the  pressure  in  stellar  atmospheres. 

THE  "FLASH"  SPECTRUM  WITHOUT  AN  ECLIPSE. 
In  designing  the  first  tower  telescope,  one  of  the 
objects  in  view  was  to  provide  a  means  of  photo- 
graphing the  "flash"  spectrum  without  the  aid 
of  a  total  eclipse.  When  the  moon  passes  between 
the  earth  and  the  sun  it  cuts  off  the  brilliant  light 
of  the  solar  disk  and  permits  the  spectrum  of  its 
gaseous  atmosphere  to  be  photographed.  The 
narrow  arc  of  light,  coming  from  this  luminous 
atmosphere  at  the  moment  when  the  sun's  disk 
is  completely  covered  by  the  dark  body  of  the 


39 

moon,  is  passed  through  a  prism,  and  the  resulting 
series  of  bright  lines  is  recorded  upon  a  sensitive 
plate.  But  the  study  of  this  "flash"  spectrum 
has  been  seriously  hampered  by  its  momentary 
visibility,  occurring  only  at  intervals  of  years.  With 
the  6o-foot  tower  telescope  the  numerous  bright 
lines  of  the  "flash"  can  be  photographed  on  any 
day  of  good  definition,  with  a  spectrograph  more 
powerful  than  those  used  in  eclipse  observations. 
In  this  way  we  not  only  get  a  marked  increase  in  the 
accuracy  of  measuring  these  lines  and  their  dis- 
placements, but  we  also  find  it  possible  to  study 
the  phenomena  of  levels  lower  than  are  attainable 
at  eclipses.  Some  remarkable  modifications  of  the 
dark-line  solar  spectrum  at  the  sun's  limb  have  also 
been  found  on  these  photographs. 

THE    I5O-FOOT   TOWER   TELESCOPE. 

The  success  of  the  first  tower  telescope  indicated 
that  the  construction  of  a  more  powerful  instru- 
ment, giving  a  larger  image  of  the  sun  (16  inches 
in  diameter),  would  be  fully  warranted.  To  secure 
the  necessary  steadiness  of  mirrors  and  lenses 
mounted  160  feet  above  the  ground,  the  plan  was 
adopted  of  incasing  each  steel  member  (leg  or 
cross-bracing)  of  a  skeleton  tower  within  the 
corresponding  hollow  member  of  another  skele- 
ton tower,  with  sufficient  clearance  to  prevent 
contact.  The  inner  tower  thus  carries  the  instru- 
ments; the  outer  tower  carries  the  dome  to  cover 
them,  while  its  members  serve  as  an  efficient 


FIG.  30. — iso-foot  Tower  Telescope. 


wind-shield.  Thus  the 
requisite  steadiness  has 
been  secured,  in  spite  of 
the  great  height  of  the 
structure.  In  other  re- 
spects the  new  tower  is 
also  a  decided  improve- 
ment, adding  still  fur- 
ther to  the  sharpness  of 
the  solar  image  during 
the  warmer  hours  and 
thus  increasing  the  du- 
ration of  the  period  of 
observation,  which  with 
this  telescope  lasts 
throughout  the  day. 
The  opening  up  of  the 
various  new  fields  of 
solar  research  taxes  the 
capacity  of  all  three 
telescopes,  each  of  which 
is  devoted  to  the  work 
for  which  it  is  best 
adapted. 

A  feature  of  the  new 
tower  telescope,  which 
is  quite  as  important  as 
the  enlarged  solar  image, 
is  the  spectrograph,now 
extended  to  a  focal 
length  of  75  feet  and 
mounted  in  a  deep  well 


FIG.  31.— Section  of  is 
'  Tower  Telescope. 


42 

beneath  the  tower.  With  this  powerful  instrument 
the  D  lines  of  sodium,  which  are  barely  separated 
with  the  standard  one-prism  spectroscope  of  the 
chemist,  are  about  1.2  inches  apart  in  the  third- 
order  spectrum.  A  photograph  of  the  solar  spec- 
trum on  this  scale,  including  the  ultra-violet  but  ex- 


FIG.  32. — Summit  of  iso-foot  Tower  Telescope. 

eluding  the  infra-red,  would  be  70  feet  long.  With 
the  aid  of  Koch's  recording  microphotometer,  the 
gain  in  precision  of  measurement  is  directly  pro- 
portional to  the  length  of  the  spectrograph.  Thus, 
as  compared  with  the  3.5  foot  Kenwood  spectro- 


43 

graph,  of  which  it  is  the  direct  successor,  the  75- 
foot  spectrograph  gives  results  fully  20  times  as 
accurate. 


FIG.  33. — Observing  Room  and  ys-foot  Spectrograph. 
THE    SUN    AS    A    MAGNET. 

What  this  means  in  practice  is  illustrated  by  a 
recent  investigation.  As  already  explained,  many 
of  the  spectrum  lines  are  split  into  two  or  more 
components  by  a  magnetic  field.  A  Nicol  prism 
and  quarter-wave  mica  plate,  placed  over  the  slit 
of  the  spectrograph,  permit  us  in  the  laboratory 
to  cut  off  either  component  at  will.  The  use  of 
a  compound  quarter-wave  plate,  made  of  narrow 
strips,  gives  the  serrated  appearance  of  the  lines 
shown  in  Fig.  22.  If  the  lines  in  the  solar 


44 

spectrum  are  similarly  affected,  and  if  the  degree 
of  their  displacement  varies  from  pole  to  equator 
as  calculation  shows  it  should  do  on  a  magnetized 
sphere,  we  may  conclude  that  the  whole  sun  is  a 
magnet.  An  extended  investigation  (rendered 
difficult  by  the  very  minute  displacements  of  the 
solar  lines,  far  too  small  to  appear  to  the  eye  in 
the  photographs)  has  led  us  to  the  conclusion  that 
the  sun  is  a  magnet,  with  its  poles  lying  at  or  near 
the  poles  of  rotation. 


FIG.  34. — D  I,ines  of  Solar  Spectrum,  with  Iodine  Absorption  Spectrum 
superposed. 

The  sun  in  this  respect  resembles  the  earth, 
which  has  long  been  known  to  be  a  magnet.  The 
general  magnetic  field  of  the  sun,  although  about 
80  times  as  intense  as  that  of  the  earth,  is  so  weak 
compared  with  the  magnetic  fields  in  sun-spots 
that  the  full  power  of  the  75-foot  spectrograph  was 
required  to  reveal  it.  As  the  sun  rotates  on  its 
axis,  it  permits  the  magnetic  phenomena  of  all 
parts  of  its  surface  to  be  studied.  Photographs 
of  the  spectra  over  a  wide  range  of  latitude  are 
therefore  made  daily,  in  order  to  provide  material 
for  charts  which  will  show  the  exact  position  of  the 


45 


magnetic  poles  and  the  intensity  of  the  field  at 
different  levels  in  the  solar  atmosphere. 

Our  interest  in  the  sun's  magnetism  is  not  con- 
fined to  the  field  of  solar  physics;  its  study  should 
aid  in  explaining  the  source  and  fluctuations  of  the 
earth's  magnetism  and  in  the  interpretation  of 
certain  stellar  phenomena.  It  is  not  improbable, 
as  Schuster  has  suggested,  that  every  star,  and 


•f-^</y:- 


FIG.  35. — Lines  of  Force  about  a  Magnetized  Sphere. 

perhaps  every  rotating  body  of  whatever  nature, 
becomes  a  magnet  through  the  fact  of  its  rotation. 
It  is  hoped  that  the  roo-inch  reflector  will  enable 
this  test  for  magnetism  to  be  applied  to  certain 
stars. 

It  may  be  well  to  add  that  at  the  distance  of  the 
earth  the  solar  magnetic  field  can  not  be  directly 
appreciable,  since  the  effect  of  one  pole  counteracts 
the  equal  and  .opposite  effect  of  the  other  pole. 


STELLAR    PROBLEMS. 

These  examples  will  suffice  to  illustrate  the 
character  of  the  solar  work  in  progress  on  Mount 
Wilson.  Let  us  now  consider  for  a  moment  the 
broader  bearing  of  these  results,  each  of  which  has 
a  wide  range  of  application.  Thousands  of  stars, 
in  the  same  stage  of  evolution  as  the  sun,  doubt- 
less exhibit  similar  phenomena,  which  are  hidden 
from  us  by  distance.  No  possible  increase  in  the 
power  of  our  telescopes,  so  far  as  can  be  judged 
from  present  knowledge,  will  ever  render  star 
images  comparable  in  size  with  the  solar  image. 
But  the  knowledge  derived  from  the  study  of  the 
sun  prepares  us  to  solve  problems  otherwise  much 
more  difficult.  For  example,  the  results  of  the 
work  on  sun-spot  spectra,  in  harmony  with  other 
phenomena,  render  it  safe  to  attribute  to  reduced 
temperature  the  bands  and  the  weakened  and 
strengthened  lines  in  the  spectra  of  Arcturus  and 
other  stars.  This  conclusion  will  assist  in  arrang- 
ing the  stars  on  a  temperature  basis,  showing  the 
gradual  changes  they  have  passed  through  in  the 
different  periods  of  their  existence.  Again,  the 
peculiar  behavior  of  certain  lines  in  the  sun  has 
recently  led  to  the  detection  of  an  interesting 
relationship  between  a  star's  spectrum  and  its 
absolute  magnitude,  which  provides  a  new  and 
very  effective  way  of  determining  stellar  distances 
and  throws  much  new  light  on  the  evolution 
problem. 


47 


FIG.  36. — The  Great  Nebula  in  Andromeda. 

We  are  thus  prepared  to  appreciate  the  intimate 
relationship  which  unites  all  phases  of  the  Observa- 
tory's work  and  determines  its  plan  of  operation. 
But  how  shall  we  enter  the  vast  sphere  of  the 
sidereal  world,  where  hundreds  of  millions  of 
objects,  exhibiting  phenomena  of  inconceivable 
magnitude  and  infinite  variety,  vie  with  one  an- 


48 

other  in  their  appeal  to  the  observer  and  tend  to 
scatter  and  exhaust  his  efforts?  The  simplest  and 
most  obvious  step  would  be  to  make  just  such  a 
study  of  the  physical  phenomena  of  selected  stars, 
representing  different  phases  of  evolution,  as  we 
are  now  making  of  the  sun.  In  spite  of  the  neces- 
sity, because  of  their  feeble  brightness,  of  basing 
our  conclusions  on  spectra  a  few  inches  long,  repre- 
senting the  combined  light  from  all  parts  of  the 
stellar  disks,  material  progress  could  be  made  in 
this  way.  But  the  importance  of  making  the 
most  effective  use  of  our  instrumental  equipment 
and  of  accomplishing  the  greatest  possible  advance 
within  a  limited  period  of  years  led  to  the  post- 
ponement of  most  of  this  work  and  the  immediate 
adoption  of  a  different  plan. 

THE    STRUCTURE    OF    THE    UNIVERSE. 

A  great  Dutch  astronomer,  gifted  with  a  power- 
ful scientific  imagination,  had  been  engaged  for 
many  years  in  the  study  of  the  structure  of  the 
universe.  The  fact  that  he  had  no  telescope  or 
other  observatory  equipment  did  not  hamper  in 
the  least  his  ambitions  or  his  successes.  On  the 
contrary,  it  led  to  his  cooperation  with  astronomers 
in  various  parts  of  the  world,  who  gladly  contrib- 
uted toward  the  realization  of  his  far-reaching 
projects.  Kapteyn's  first  ally  was  the  late  Sir 
David  Gill,  Astronomer  Royal  at  the  Cape  of  Good 
Hope,  who  had  photographed  the  whole  of  the 
southern  heavens.  After  twelve  years  of  patient 


49 

labor  in  measuring  the  positions  of  454,875  stars 
on  these  photographs,  Kapteyn  turned  his  atten- 
tion to  the  study  of  stellar  motions.  He  found, 
in  brief,  that  all  the  stars  whose  motions  were 
known  belonged  in  one  or  the  other  of  two  great 
intersecting  streams,  which  have  been  moving 
through  space  since  time  immemorial.  Projected 
back  into  the  positions  which  they  occupied  in  the 
remote  past,  these  stars  represent  two  great  sys- 
tems, which  drew  toward  one  another,  interpene- 
trated, and  now  continue  toward  their  unknown 
goals. 

This  impressive  result,  with  its  strong  appeal  to 
imaginations  curious  as  to  the  past  or  the  future, 
was  but  a  single  step  in  Kapteyn's  progress.  His 
plans  involved  the  study  of  great  numbers  of 
stars,  uniformly  distributed  over  the  heavens  and 
embracing  questions  of  brightness,  of  distance, 
and  of  motion.  Through  the  cooperation  of  many 
astronomers  the  necessary  measurements  to  solve 
these  questions  will  ultimately  be  obtained.  But 
the  great  light-collecting  power  of  our  6o-inch 
reflector  and  the  facilities  provided  for  its  use  indi- 
cated that  it  might  prove  of  special  service  in  the 
realization  of  Kapteyn's  hopes.  Could  this  help 
be  given  without  loss  to  our  own  enterprise? 

STELLAR    EVOLUTION. 

For  it  will  be  observed  that  Kapteyn's  project  dif- 
fered materially  from  our  own.  He  was  occupied 
with  such  questions  as  the  limits  of  the  universe, 


50 

the  number  of  the  stars,  their  distribution  in  space, 
their  association  in  groups,  and  their  common 
motions.  He  was  not  directly  concerned  with 
those  studies  of  physical  condition  and  of  evolu- 
tional progress  in  which  we  hoped  to  aid  in  tracing 


FIG.  37. — The  Milky  Way  near  6  Ophiuchi. 

the  life-history  of  stars  and  stellar  systems  from 
their  birth  to  their  decay.  Could  we  afford  to 
postpone  some  of  our  own  investigations,  at  first 
sight  very  different  from  his,  even  for  the  very 
important  purpose  of  advancing  his  great  under- 
taking? 


It  appeared  on  reflection  that  the  surest  way  to 
accomplish  our  own  special  object  was  to  coop- 
erate in  the  closest  possible  way  with  Kapteyn, 
even  to  the  extent  of  devoting  a  large  share  of  the 
working-time  of  our  instruments  to  the  study  of 
his  problems.  The  physical  development  of  stars 
may  depend  upon  just  such  association  in  systems 
as  the  discovery  of  star-streams  has  disclosed,  and 
many  questions  might  long  escape  answer  if  at- 
tacked only  from  a  single  viewpoint.  Irr  short, 
close  cooperation  should  prove  mutually  advan- 
tageous, contributing  in  material  measure  toward 
the  solution  of  what,  after  all,  is  but  a  single  great 
problem.  How  manifestly  the  annual  visits  of 
Kapteyn  to  Mount  Wilson  have  aided  progress 
toward  our  original  goal  will  appear  in  the  sequel. 

INSTRUMENTAL    POSSIBILITIES. 

And  now  a  word  as  to  instrumental  means. 
Here  we  may  advance  in  two  ways:  (i)  as  we  have 
seen,  by  the  use  of  methods  and  the  adaptation  of 
principles,  borrowed  in  the  main  from  the  phys- 
icist, and  (2)  by  increase  in  telescopic  power, 
derived  from  greater  optical  aperture  and  greater 
perfection  of  optical  and  mechanical  construction. 
Mere  size  is  of  no  moment,  unless  supported  by 
corresponding  precision  of  parts.  Lord  Rosse's 
6-foot  reflector,  built  before  the  development  of 
modern  machine  tools,  was  less  efficient  than  a 
12-inch  telescope  of  the  present  day.  It  is  true 
that  photography  is  mainly  responsible  for  the 


gain  in  observational  method,  but  the  design  and 
construction  of  Lord  Rosse's  instrument  would 
have  precluded  his  use  of  the  sensitive  plate.  The 
types  of  telescopes  adopted  for  our  solar  studies 
have  already  been  described.  Let  us  now  consider 
the  very  different  needs  of  stellar  observations, 
where  the  first  requirement  in  many  classes  of 
work  is  the  collection  of  the  greatest  possible 
amount  of  light. 


FIG.  38. — Mirror  Combinations  in  6o-inch  Reflector. 

Most  of  the  astronomical  telescopes  in  use  are 
refractors,  consisting  of  a  lens,  through  which  the 
rays  from  a  star  pass,  to  be  united  in  an  image  at 
the  lower  end  of  the  telescope  tube.  A  reflecting 
telescope,  on  the  other  hand,  consists  of  a  silvered 
concave  mirror,  lying  at  the  lower  end  of  a  tube, 
the  upper  end  of  which  is  open.  The  parallel 


S3 

rays  of  light,  after  falling  on  the  mirror,  converge 
to  the  focal  point,  which  in  this  case  is  at  the  upper 
end  of  the  tube  (Fig.  38).  The  photographic  plate 
may  be  fixed  in  the  axis  of  the  tube  or  placed  at 
one  side,  where  it  receives  the  image  after  reflec- 
tion from  a  plane  mirror  mounted  diagonally.  In 
another  arrangement  a  convex  mirror  takes  the 
place  of  the  diagonal  plane  and  sends  the  rays  back 
toward  the  base  of  the  tube,  where  they  are  again 
reflected  by  a  plane  mirror,  either  to  a  point  on 
one  side  of  the  tube  or  down  through  the  hollow 
polar  axis,  to  unite  in  an  image  on  the  slit  of  a 
powerful  spectrograph  mounted  in  a  constant- 
temperature  chamber. 


Fir,.   39. — 6o-inch   Reflector,   showing  Stellar  Spectrograph  on  Tube. 


54 


THE    6O-INCH    REFLECTOR. 

In  modern  astrophysical  research  the  eye  has 
given  place  to  the  photographic  plate,  which  in 
almost  all  cases  records  much  more  than  the  eye 
can  perceive.  As  the  reflector  is  especially  adapted 
for  celestial  photography,  this  form  of  telescope 
was  chosen  for  our  stellar  work,  for  which  the  Snow 
and  tower  telescopes  are  not  suitable.  A  5-foot 
mirror,  already  ground  and  partly  polished  at  the 


L 


FIG.  40. — Sectional  Drawing  of  6o-inch  Reflector  and  Dome. 

Yerkes  Observatory,  was  acquired  for  use  on 
Mount  Wilson  and  brought  to  a  perfect  figure  in 
our  optical  shop  in  Pasadena.  The  heavy  parts  of 
the  mounting,  some  of  which  weigh  5  tons  each, 
were  made  at  the  Union  Iron  Works  in  San  Fran- 
cisco; while  all  of  the  more  exacting  work,  requiring 
greater  precision,  including  the  assembling  of  the 
instrument,  the  cutting  of  the  teeth  of  the  lo-foot 


55 

worm-gear  (when  mounted  in  place  on  the  polar 
axis),  and  the  construction  of  the  driving-clock, 
was  done  by  our  own  mechanicians.  A  necessary 
part  of  the  undertaking  was  the  building  of  a 
mountain  road  over  9  miles  long,  the  transporta- 
tion of  the  telescope  and  the  steel  building  with 
its  revolving  dome  to  the  summit,  and  their  erec- 
tion by  our  construction  corps. 


FIG.  41. — Great  Cluster  in  Hercules. 

Five  years  of  work  with  this  telescope  have 
brought  out  all  of  its  admirable  qualities  and  pro- 
vided a  rich  store  of  photographs  for  the  study  of 
stellar  evolution.  These  are  of  the  most  varied 
character,  including  star  clusters  (showing  in  one 
case  some  30,000  stars),  nebulae,  stellar  spectra, 
and  occasionally  the  moon,  planets,  and  comets, 
though  the  three  latter  classes  of  objects  are  not 
included  in  our  present  scheme  of  research. 


FIG.  42. — Spiral  Nebula  M  81  Ursae  Majoris. 
THE    NEBULAE. 

Hypotheses  of  stellar  evolution  usually  assume 
that  the  stars  and  stellar  systems  originate  in  the 
nebulae,  of  which  many  excellent  photographs,, 
showing  much  new  and  remarkable  structure, 
have  been  obtained  on  Mount  Wilson.  These 
cloud-like  objects,  many  of  which  are  shown 
by  the  spectroscope  to  consist  of  glowing  gases, 
occur  in  the  widest  variety  of  form.  The 
vast  majority,  however,  are  of  definite  spiral 
structure.  Several  nights  of  each  month  are  de- 
voted to  a  systematic  photographic  survey  of  the 


57 


FIG.  43. — Spiral  Nebula  M  101  Ursae  Majoris. 

small  nebulae,  which  have  been  catalogued  in 
great  numbers  by  Herschel  and  others,  but  never 
studied,  except  in  a  moderate  number  of  cases, 
under  adequate  telescopic  power;  in  fact,  in  most 
instances  the  existing  (visual)  records  give  no  con- 
ception whatever  of  the  true  form  of  these  curious 
objects.  At  the  same  time  the  spectra  of  many  of 
these  nebulae  are  being  photographed. 

Keeler's  discovery,  that  there  are  at  least  100,000 
spiral  nebulae,  left  no  doubt  of  their  fundamental 
importance  in  the  scheme  of  stellar  evolution.  The 
development  represented  by  their  spiral  forms  is 


FIG.  44.— Spiral  Nebula  M  33  Trianguli, 

on  an  incomparably  greater  scale  than  that  of  our 
own  solar  system,  which  would  shrink  to  the  di- 
mensions of  a  pin-point  if  seen  at  the  same  dis- 
tance. This  distance  is  so  great  that  in  spite  of  the 
realistic  appearance  of  motion  in  these  spirals,  no 
evidence  of  change  in  form  has  yet  been  detected. 
But  Slipher  has  found  displacements  of  their 
spectral  lines  indicating  rotational  motion,  and 


59 

the  planetesimal  hypothesis  offers  a  possible  means 
of  accounting  for  their  origin  and  development. 
We  are  fortunate  in  being  able  to  cooperate  with 
Chamberlin  and  Moulton,  the  authors  of  this 
hypothesis,  in  a  study  of  the  structure  of  spiral 
nebulae.  The  small  spirals  are  of  peculiar  interest, 
and  here  the  loo-inch  telescope  should  be  of  service 
in  extending  our  knowledge  of  the  structural  de- 
tails of  objects  now  barely  recognizable  as  spiral 
in  form. 

MAGNITUDE    OF    THE    UNIVERSE. 

In  our  study  of  the  sun  as  a  typical  star  and  in 
this  investigation  of  nebulae  as  the  probable  source 
of  stellar  existence,  we  are  thus  engaged  on  two 
phases  of  the  evolution  problem.  Let  us  now  see 
how  the  use  of  the  6o-inch  reflector  in  cooperation 
with  Kapteyn  has  contributed  in  the  same  direc- 
tion. A  question  of  prime  importance  in  astron- 
omy is  the  magnitude  of  the  universe.  As  our 
telescopes  increase  in  power  we  reach  farther  and 
farther  into  space.  As  the  sphere  of  vision  en- 
larges, each  increase  in  its  diameter  must  mean 
the  addition  of  a  larger  number  of  stars  than  the 
previous  equal  increase  produced,  because  it  in- 
volves the  addition  of  a  larger  volume  of  space. 
This  reasoning  is  based  on  the  assumption  that 
the  stars  are  distributed  nearly  uniformly  through 
space,  at  least  as  far  as  we  can  penetrate  it,  and 
that  no  light  is  lost  in  transmission. 

In  practice  we  find  that  each  successive  advance 
into  a  region  of  more  distant,  and  therefore  (on 


6o 


FIG.  45. — Spiral  Nebula  HV  24  Comae  Berenices. 

the  average)  fainter,  stars  does  mean  the  addition 
of  a  greater  number;  but  toward  the  outer  boun- 
daries of  the  known  sphere  the  increase  is  less  rapid 
than  would  be  anticipated.  Kapteyn  pointed  out 
that  this  might  be  due,  at  least  in  part,  not  to  an 
actual  thinning  out  of  the  stars  near  the  outer 
boundary  of  the  sphere,  but  to  scattering  of  light 
by  minute  particles  distributed  at  wide  intervals 
through  space.  What  would  be  the  result?  We 
know  that  the  sun  appears  red  at  sunset,  or  when 
seen  through  smoke,  because  the  blue  light  is 
intercepted  and  scattered  by  the  minute  floating 


6i 


FIG.  46. — Milky  Way,  showing  Nebula  surrounding  p  Ophiuchi. 

particles  more  than  the  red  light.  For  the  same 
reason  the  very  distant  stars  should  appear  redder 
than  the  nearer  ones,  if  there  is  similar  scattering 
in  space.  It  is  therefore  necessary  to  determine, 
by  several  independent  methods,  the  proportion 
of  red  and  blue  light  sent  us  by  near  and  distant 
stars. 

THE    BRIGHTNESS    OF    THE    STARS. 

A  knowledge  of  the  relative  intensity  of  the  light 
of  the  different  stars,  from  brightest  to  faintest, 
is  essential  in  statistical  investigations  of  their 
distances,  luminosities,  and  distribution  in  space. 


62 


FIG.  47. — Spiral  Nebula  N.  G.  C.  4736. 

The  brightness  is  expressed  in  quantities  called 
magnitudes,  ranging  from  about  I  for  the  brightest 
stars  to  about  20  for  the  faintest  objects  registered 
in  4  hours'  exposure  with  the  6o-inch  reflector. 
A  star  i  magnitude  brighter  than  another  gives 


FIG.  48. — Spiral  Nebula  M  51  Canum  Venaticorum. 

light  about  2.5  times  as  intense.  Thus  an  interval 
of  5  magnitudes  corresponds  to  an  intensity  ratio 
of  i  to  100,  while  the  most  brilliant  stars  are  about 
100,000,000  times  as  intense  as  the  faintest  photo- 
graphed with  the  6o-inch  telescope. 


64 

This  great  range  of  intensity  seriously  compli- 
cates the  determination  of  the  magnitude  scale, 
but  photographic  methods  have  been  developed 
with  the  6o-inch  reflector,  and  applied  to  a  deter- 
mination of  standard  magnitudes  for  all  the  stars 
of  Pickering's  North  Polar  Sequence  and  numerous 
other  faint  stars  near  the  Pole.  The  scale  thus 
established,  over  a  range  of  about  ij\  magnitudes, 
probably  represents  satisfactorily  the  degree  of 
homogeneity  attainable  by  the  use  of  uniform 
methods  employed  with  a  single  instrument. 

For  the  ready  determination  of  stellar  magni- 
tudes over  the  whole  sky,  standard  magnitudes 
on  an  absolute  scale,  of  a  considerable  number  of 
uniformly  distributed  stars,  must  be  measured. 
To  this  end  observations  of  such  standard  stars 
to  the  seventeenth  magnitude  in  each  of  Kapteyn's 
115  Selected  Areas  on  and  north  of  the  celestial 
equator  are  now  in  progress. 

Magnitudes  measured  on  ordinary  photographic 
plates  give  the  intensity  of  the  blue  and  violet  light 
of  the  stars.  By  using  plates  sensitive  to  red  or 
yellow,  behind  a  yellow  glass  filter,  the  resulting 
"photovisual"  magnitudes  furnish  a  measure  of 
the  intensity  of  the  red  or  yellow  light.  As  the 
relative  intensity  of  the  blue  and  red  light  for  any 
object  depends  upon  its  color,  a  comparison  of  the 
photographic  and  photovisual  magnitudes  gives  at 
once  a  measure  of  the  color.  The  method  is  of 
importance,  as  it  can  be  applied  to  the  faintest 
stars  that  can  be  photographed. 


A  comparison  of  this  kind,  made  for  about  300 
faint  stars  near  the  pole  and  about  300  additional 
stars  in  two  other  regions,  shows  clearly  that  there 
is  a  gradual  change  in  color  with  brightness.  The 
fainter  stars,  on  the  average,  are  redder  than  the 
brighter  ones. 

But  while  this  result  is  just  what  would  be 
expected  on  the  basis  of  Kapteyn's  reasoning,  other 
possible  explanations  must  not  be  overlooked. 
The  increased  redness  of  the  fainter  stars  may  be 
due  to  a  gradually  increasing  preponderance  of 
late  spectral  types  (old  stars)  with  decreasing 
brightness;  it  may  be  (as  Kapteyn  pointed  out  in 
his  papers)  a  consequence  of  absolute  luminosity, 
which  for  the  faint  stars  is  less,  on  the  average, 
than  for  the  brighter  objects;  or  it  may  be  due  to 
scattering  of  light  in  space.  Perhaps  all  of  these 
possibilities  enter  in  some  degree.  We  shall  soon 
see  how  the  study  of  stellar  spectra  bears  upon 
this  problem. 

STELLAR   MOTIONS. 

The  extension  of  Kapteyn's  study  of  star- 
streaming  involves  the  measurement  of  the  ve- 
locity, toward  or  away  from  the  earth,  of  a  great 
number  of  stars.  These  velocities  are  determined 
by  means  of  the  spectrograph  shown  attached  to 
the  telescope  tube  (Fig.  39).  Side  by  side  on  the 
photographic  plate,  the  spectrum  of  a  star  and  the 
standard  lines  in  the  spectrum  of  iron  or  titanium 
are  recorded.  If  the  titanium  or  iron  lines  in  the 
star  are  shifted  toward  the  red,  with  reference  to 


66 

the  corresponding  lines  of  the  comparison  spec- 
trum, we  know  that  the  star  is  moving  away  from 
the  earth  at  a  velocity  proportional  to  the  amount 
of  the  shift.  Displacement  toward  the  violet 
means  motion  toward  the  earth.  This  method, 
first  applied  on  a  large  and  comprehensive  plan  by 
Campbell,  of  the  Lick  Observatory,  gave  the  ve- 
locities of  1,500  stars  and  yielded  many  conclusions 
of  great  importance.  The  6o-inch  reflector  has 


I 

FIG.  49. — Spectrum  of  Lalande  1966,  showing  velocity  of  325  km.  per 
second  toward  the  Earth.  Position  of  lines  in  stellar  spectrum 
and  of  corresponding  lines  in  comparison  spectrum  indicated  at 
bottom. 

enabled  us  to  extend  such  measures  to  many  fainter 
stars.  A  discussion  of  the  resulting  velocities 
shows  that  Kapteyn's  two  star-streams  extend 
into  space  much  farther  than  the  original  data 
(for  nearer  stars)  permitted  them  to  be  traced. 
Thus  the  view  that  the  main  body  of  the  universe 
is  constituted  of  these  streams  receives  added 
support. 

STARS    OF    HIGH    VELOCITY. 

Kapteyn  and  Campbell  independently  found 
that  the  radial  velocities  of  the  stars,  corrected  for 
the  sun's  motion,  range  from  about  6  kilometers 


67 

per  second  for  the  early  type  (young)  stars  to 
about  20  kilometers  per  second  for  the  late  type 
(old)  stars.  Boss,  whose  meridian-circle  obser- 
vations in  the  northern  and  southern  hemispheres 
have  determined  the  positions  of  numerous  stars 
with  the  highest  precision,  deduced  a  similar  con- 
clusion from  his  studies  of  stellar  motions  perpen- 
dicular to  the  line  of  sight.  The  meaning  of  this 
remarkable  variation  of  velocity  with  type  is  not 
yet  understood  and  it  will  remain  for  future  re- 
searches on  the  fainter  stars  to  develop  its  full 
significance. 

Although  the  average  velocities  of  stars  are  thus 
found  to  be  moderate,  notable  exceptions  are  fre- 
quently encountered  in  the  work  with  the  6o-inch 
reflector.  Among  the  high  velocities  in  space  thus 
far  found  on  Mount  Wilson,  cases  of  141,  150,  179, 
233,  316,  and  320  kilometers  per  second  may  be 
mentioned.  The  last-named  is  the  highest  velocity 
of  translation  yet  found  for  any  star.  At  these 
speeds  the  attraction  of  the  entire  known  stellar 
system  would  be  wholly  insufficient  to  check  the 
star  in  its  flight  into  unknown  regions  of  space. 

SPECTROSCOPIC    BINARIES. 

Pickering  was  the  first  to  discover  a  to-and-fro 
shift,  in  a  definite  period,  of  the  lines  in  the  spectra 
of  certain  stars.  He  attributed  it  to  orbital  mo- 
tion, and  thus  the  possibility  of  detecting  "spec- 
troscopic  binaries"  was  shown.  These-are  double 
stars  lying  too  close  together  to  be  separately  dis- 


68 

tinguished  with  any  telescope,  but  betraying  their 
existence  by  the  effect  of  their  motion.  Usually 
one  member  of  the  pair  is  too  faint  to  give  any 
spectrum.  The  lines  in  the  spectrum  of  the  visible 
star  then  swing  back  and  forth,  toward  the  violet 
as  the  star  approaches  the  earth,  toward  the  red 
as  it  recedes.  When  both  stars  are  nearly  equal 
in  brightness  both  spectra  appear  superposed  and 
the  lines,  which  are  single  when  the  two  stars  are 
moving  across  the  line  of  sight,  become  double 
when  one  is  approaching  and  the  other  receding. 

Thus  far  115  spectroscopic  binaries  have  been 
discovered  on  Mount  Wilson,  or  about  I  in  4  of 
all  the  stars  whose  velocities  have  been  measured. 
In  some  interesting  cases  of  rapid  motion  the  rela- 
tive orbital  velocities  of  the  components  range  from 
104  to  367  kilometers  per  second. 

TEMPORARY    STARS. 

Among  the  most  remarkable  phenomena  of  the 
heavens  are  the  "new"  or  temporary  stars,  which 
burst  out  into  sudden  brilliancy  and  gradually 
fade  into  extreme  faintness.  With  the  6o-inch 
reflector  it  has  been  possible  to  photograph  the 
spectra  of  some  interesting  "Novae"  which  ap- 
peared several  years  ago  and  are  now  very  faint. 
These  include  Nova  Aurigae,  which  was  discovered 
in  1891  and  is  now  of  magnitude  13.5;  Nova 
Persei  of  1901,  magnitude  12.0;  Nova  Lacertae  of 
1910,  magnitude  13;  and  Nova  Geminorum  No.  2 
of  1912,  magnitude  10.  For  such  faint  objects 


69 

the  exposures  required  in  photographing  the  spec- 
trum ranged  from  2  to  16  hours. 

The  most  plausible  hypothesis  of  temporary 
stars  accounts  for  their  rapid  brightening  on  the 
supposition  that  a  faint  star  suddenly  plunges  into 
a  gaseous  nebula.  After  passing  through  a  remark- 
able series  of  changes,  the  spectra  of  Novae  have 
usually  been  supposed  to  correspond  closely,  in 
the  last  visible  stage  of  their  existence,  with  the 
spectra  of  nebulae.  The  Mount  Wilson  results,  in 
harmony  with  an  observation  by  Hartmann,  show 
that  after  the  lapse  of  years  the  characteristic  lines 
of  the  nebular  spectrum  disappear  completely, 
at  least  in  some  cases,  as  though  the  star  had 
finally  passed  out  of  the  nebula  which  caused  its 
outburst  of  light.  If  the  above  hypothesis  is  cor- 
rect, the  temporary  brightness  of  Novae  resembles 
that  of  meteorites,  which  are  kindled  into  brilliancy 
when  passing  through  the  earth's  atmosphere. 

It  also  appears  probable  that  at  least  a  portion 
of  the  remarkable  Wolf-Rayet  stars,  most  of  which 
lie  in  the  Milky  Way  (where  practically  all  Novae 
occur),  are  temporary  stars  in  the  later  stages  of 
their  existence. 

VARIABLE    STARS. 

An  interesting  result  is  derived  from  a  study  of 
the  color  of  a  certain  variable  star,  RR  Draconis, 
one  of  the  revolving  components  of  which  com- 
pletely eclipses  the  other  at  the  time  of  minimum 
brightness.  It  is  found  that  the  faint  companion, 


70 

which  is  the  larger  of  the  two  stars,  is  redder  than 
the  bright  central  object.  If  we  may  accept,  as 
generally  valid,  the  result  that  for  all  systems  of 
known  mass-ratio  the  brighter  object  is  invariably 
the  more  massive,  it  follows  that  the  fainter  and 
redder  companion  of  RR  Draconis  is  of  a  much 
lower  density  than  the  brighter  object.  Further, 
if  the  redder  color  really  represents  more  advanced 
spectral  type,  commonly  associated  with  a  later 
stage  of  development,  we  should  have  the  unex- 
pected case  of  a  star  of  low  density  associated  with 
a  less-developed  star  of  higher  density.  As  this 
is  contrary  to  the  usual  view  that  a  star's  density 
increases  with  its  age,  further  investigations  of 
such  systems  may  prove  of  importance  in  the 
study  of  stellar  evolution. 

LIGHT-SCATTERING    IN    SPACE. 

The  collection  of  nearly  4,000  photographs  of 
stellar  spectra  thus  far  obtained  on  Mount  Wilson 
is  available  for  many  classes  of  work.  It  affords 
Kapteyn  material  for  the  further  investigation  of 
star-streaming,  which  is  now  being  studied  with 
reference  to  the  association  in  streams  of  stars  in 
different  stages  of  physical  development;  but  it 
also  permits  many  other  problems  to  be  attacked. 
One  of  these  is  the  question  of  light-scattering  in 
space.  We  have  already  seen  that  the  fainter 
stars  are  redder  than  the  brighter  ones,  but  the 
meaning  of  this  result  is  not  yet  certain. 

One  of  the  methods  used  to  determine  the  rela- 
tive colors  of  near  and  distant  stars  was  to  photo- 


graph  their  spectra  side  by  side  on  a  single  plate. 
Since  blue  and  violet  light  is  weakened  by  scatter- 
ing more  than  red,  we  should  expect  the  violet  part 
of  the  spectrum  of  the  more  distant  star  to  be 
fainter  than  that  of  the  nearer  star.  In  general, 
this  proved  to  be  the  case. 

Many  spectra  of  near  and  distant  stars  were 
then  selected  from  our  collection  of  photographs 
and  divided  into  groups,  each  representing  a  dif- 
ferent stage  of  evolution.  That  is  to  say,  in 
terms  of  our  present  view  of  stellar  ages,  young, 


FIG.  50. — Weakness  in  Violet  of  the  Spectra  of  Distant  Stars. 
Compare  middle  spectrum  of  each  group  with  those  above  and 
below. 

middle-aged,  and  old  stars  were  placed  in  separate 
groups.  It  was  found  that  in  all  of  the  groups, 
with  one  possible  exception,  the  more  distant  stars 
are  redder  than  the  nearer  ones,  but  that  the  dif- 
ference in  color  also  depends  in  some  way  upon  the 
age  of  the  star. 

Investigations  are  now  in  progress  to  explain 
this,  but  the  advantage  of  dealing  with  Kapteyn's 
problem  of  distance  from  the  astrophysical  stand- 
point is  already  evident.  On  the  one  hand,  an 
influence  of  physical  development  has  been  found 


72 

which  must  be  considered  before  any  final  con- 
clusions can  be  drawn  regarding  scattering  in  space 
or  its  bearing  on  the  extent  of  the  universe.  On 
the  other  hand,  the  study  of  a  problem  which 
might  seem  to  have  little  to  do  with  the  question 
of  stellar  evolution  has  brought  to  light  a  physical 
phenomenon  which  may  prove  to  be  an  important 
factor  in  the  explanation  of  stellar  development. 
The  first  application  of  these  results  has  provided 
a  new  means  of  determining  stellar  distances. 

A  NEW  MEASURE  OF  STELLAR  DISTANCES. 

We  have  already  spoken  of  the  apparent  mag- 
nitudes of  the  Stars.  Their  absolute  magnitudes, 
on  the  other  hand,  are  the  magnitudes  they  would 
have  if  they  were  all  at  the  same  standard  distance 
from  the  earth.  To  calculate  these,  we  must  know 
the  distances  of  the  stars.  Conversely,  if  we  can 
obtain  the  absolute  magnitude  of  a  star  in  some 
other  way,  we  can  determine  its  distance  by  com- 
paring this  with  its  apparent  magnitude. 

A  knowledge  of  the  peculiarities  of  certain  lines 
in  the  sun's  spectrum,  derived  from  the  solar  in- 
vestigations referred  to  above,  suggested  that  their 
relative  intensities  be  determined  in  the  spectra 
of  a  list  of  stars  of  known  absolute  magnitude. 
This  brought  out  a  surprisingly  close  correspond- 
ence between  these  relative  intensities  and  the 
absolute  magnitudes.  Hence,  by  determining  the 
ratio  of  these  intensities  and  the  apparent  magni- 
tude for  any  star  of  this  class,  we  can  at  once 


73 

calculate  its  distance.  If  this  method  proves  to 
be  generally  applicable  it  will  give  a  means  of 
finding  the  distances  of  stars  too  remote  to  be 
measured  by  other  means.  From  a  physical  point 
of  view  it  is  also  of  value,  because  of  its  bearing  on 
the  constitution  of  stellar  atmospheres. 


FIG.  51. — Dome  of  6o-inch  Reflector. 
OTHER  RESEARCHES  IN    PROGRESS. 

A  detailed  account  of  the  investigations  made 
or  in  progress  would  include  many  additional  sub- 
jects, such  as  the  measurement  of  the  brightness 
of  the  night  sky,  the  study  of  the  spectra  of 
nebulae,  star  clusters,  the  Zodiacal  Light,  and  the 
Milky  Way,  the  determination  of  standards  of 
wave-length  for  the  use  of  spectroscopists,  the 


74 

photography  of  Halley's  comet,  statistical  studies 
of  solar  prominences,  sun-spots,  and  flocculi,  etc. 
(See  "Published  Papers  of  the  Observatory,"  page 
92.)  Enough  has  been  said,  however,  to  give  an 
idea  of  the  character  of  the  Observatory's  work  and 
the  nature  of  the  instruments  employed.  In  addi- 
tion to  the  four  telescopes  already  mentioned,  a 
6-inch  refractor  and  a  lo-inch  photographic  tele- 
scope of  the  portrait-lens  type  are  soon  to  be 
provided. 

THE  WORK  OF  INTERPRETATION. 

The  interpretation  of  the  varied  phenomena  re- 
corded on  astronomical  photographs  is  the  most 
important  phase  of  the  Observatory's  work.  At 
first  thought  it  might  seem  that  a  good  photograph 
of  a  celestial  object  would  represent  the  chief  aim 
of  the  astronomer.  In  fact,  however,  it  is  only  a 
first  step,  since  the  information  it  contains  does 
not  often  lie  on  the  surface.  It  is  the  task  of  the 
investigator,  not  merely  to  take  photographs,  but 
to  interpret  them.  To  return  to  a  former  simile, 
he  is  in  the  position  of  Young  and  Champollion 
when  attempting  to  read  the  hieroglyphic  inscrip- 
tion of  the  Rosetta  Stone,  or,  more  often,  in  that  of 
present-day  archeologists  when  facing  the  seem- 
ingly impossible  task  of  deciphering  the  unknown 
characters  of  the  Minoan  civilization  in  Crete.  If, 
as  in  the  latter  case,  the  duplicate  inscription  in  a 
known  tongue  is  lacking,  much  searching  and 
many  failures  may  precede  the  discovery  of  the 
hidden  meaning  of  the  document. 


FIG.  52. — Pasadena  from  Mount  Wilson. 


76 

Let  us  suppose  that  we  are  dealing  with  the 
spectrum  of  a  star.  Side  by  side  on  the  photo- 
graphic plate  we  have  the  stellar  spectrum  and 
that  of  a  spark  between  poles  of  iron  or  titanium. 
By  measuring  these  lines  with  a  suitable  instru- 
ment, we  obtain  their  positions  within  a  few  thou- 
sandths of  a  millimeter.  When  the  positions  of 
hundreds  or,  as  in  some  cases,  thousands  of  lines 
must  be  determined  with  the  highest  possible  pre- 
cision, it  is  evident  that  the  measurer  must  be  both 
painstaking  and  persistent. 

Next  comes  the  work  of  computation.  The 
measured  distances  between  the  standard  (spark) 
lines  and  the  corresponding  lines  of  the  star  must 
first  be  converted  from  fractions  of  a  millimeter 
into  fractions  of  an  angstrom  (the  international 
unit  of  wave-length  in  spectroscopic  work).  When 
this  has  been  done,  a  short  computation  gives  the 
velocity  of  the  star's  motion  in  the  direction  of  the 
observer.  After  computing  and  deducting  the 
velocity  of  the  observer's  motion  (due  to  the  diurnal 
rotation  of  the  earth  and  its  revolution  about  the 
sun),  the  star's  motion  with  respect  to  the  sun  is 
determined. 

From  this  point  one  may  go  on,  according  to 
the  requirements  of  the  work  in  hand,  to  identify 
all  of  the  star's  lines  with  those  of  known  chemical 
elements  or  compounds;  to  account  for  minute 
displacements  of  the  lines  from  their  normal  places, 
as  the  result  of  pressure  or  electrical  excitation  or 
some  other  condition  in  the  star's  atmosphere; 


FIG.  53. — Telephoto  View  of  Pasadena  from  Mount  Wilson, 
showing  Observatory  Buildings. 


78 

to  measure  their  relative  intensities  and  compare 
them  with  the  known  effects  of  temperature 
change,  etc.  Some  other  methods  of  interpreta- 
tion have  already  been  described  in  our  account  of 
solar  phenomena. 

The  collection  of  instruments  used  in  the  Pasa- 
dena office-building  for  the  study  of  the  photo- 


FIG.  54.- — Pasadena  Office-Building. 

graphs  includes  measuring-machines  of  various 
types;  visual  photometers,  for  determining  the 
density  of  the  image  (and  hence  the  brightness  of 
the  object);  Koch's  registering  microphotometer; 
a  Zeiss  stereocomparator,  for  the  accurate  com- 
parison of  two  photographs  of  the  same  object;  the 
heliomicrometer,  a  combined  measuring  and  cal- 
culating machine  for  determining  the  latitude  and 
longitude  of  objects  on  the  sun;  several  calculating 


79 


FIG.  55. — The  Heliomicrometer. 

machines  for  rapid  addition,  multiplication,  and 
division;  and  other  devices  for  special  purposes. 
Most  of  these  instruments  were  made  in  our  own 
shop,  from  working  drawings  prepared  by  our 
draftsmen. 

Beyond  this  process  of  interpretation  lies  the 
extensive  work  of  correlation  and  generalization, 


8o 

which  seeks  to  express  a  wide  range  of  phenomena 
under  a  single  mathematical  law.  As  already  in- 
dicated, the  policy  of  the  Observatory  is  to  enlist 
in  this  endeavor  the  ablest  mathematical  astron- 
omers and  physicists,  of  Europe  or  America,  whose 
cooperation  can  be  secured. 


FIG.  56. — Large  Measuring  Machine  for  Solar  Spectra. 
INSTRUMENT  AND    OPTICAL   SHOPS. 

The  equipment  of  the  instrument  and  optical 
shops  has  grown  steadily  with  the  demands  of  the 
work  in  progress.  Our  requirements  range  from 
the  accurate  cutting  of  the  teeth  of  worm-gears  14 
feet  in  diameter  down  to  the  smallest  attachments 
of  microscopes,  galvanometers,  and  other  delicate 


8i 


instruments.  The  mounting  of  the  loo-inch  re- 
flector is  making  exceptional  demands  upon  our 
instrument  makers  and  their  shop  equipment, 
which  has  recently  been  enlarged  to  meet  the  needs 
of  this  telescope  and  the  still  more  exacting  re- 
quirements of  a  machine  for  ruling  diffraction 
gratings. 


FIG.  57. — Machine  Shop  in  Pasadena. 

The  success  of  modern  spectroscopic  investi- 
gation depends  largely  upon  the  size  and  excel- 
ence  of  the  optical  gratings  available.  These 
polished  plates  of  speculum  metal,  ruled  with 
about  15,000  lines  to  the  inch,  have  reached  a  high 
degreee  of  perfection  through  the  skill  of  Rowland, 
Michelson,  and  Anderson.  But  certain  types  of 
gratings  required  for  our  special  work  can  not  be 


82 


obtained  from  existing  sources,  and  we  are  accord- 
ingly attempting  to  overcome  the  peculiar  diffi- 
culties involved  in  making  them.  The  instrument 
shop  and  the  underground  constant-temperature 
chamber  provided  for  this  purpose  are  in  the 
Pasadena  office-building,  where  the  ruling-machine 
is  now  being  completed  and  tested. 


FIG.  58. — Grinding  Cross  Ways  of  Ruling  Machine  Bed  on  Large  Planer. 

The  larger  instrument-shop,  in  an  adjoining 
building,  contains  a  good  collection  of  machine 
tools  of  the  best  types,  and  the  optical  shop  is 
equipped  for  grinding  and  polishing  plane  and 
curved  surfaces  of  various  dimensions.  Here  the 
loo-inch  mirror  for  a  large  reflecting  telescope  is 
now  being  figured. 


THE    IOO-INCH    REFLECTOR. 

To  the  unaided  vision,  about  5,000 stars  would  be 
visible  on  a  clear  night  in  the  entire  sky.  Accord- 
ing to  a  recent  estimate  by  Chapman  and  Melotte 
the  heavens  contain  about  219,000,000  stars, 
brighter  than  the  twentieth  magnitude,  which  are 
within  the  range  of  our  6o-inch  reflector.  If  the 
indications  afforded  by  Chapman's  figures  can  be 


FIG.  59. — loo-inch  Disk  on  Grinding  Machine  turned  into  vertical 
position. 

applied  to  fainter  objects,  there  is  reason  to  hope 
that  a  loo-inch  telescope  would  add  nearly  100,- 
000,000  still  fainter  stars,  many  of  them  lying  be- 
yond the  boundary  of  the  universe  as  at  present 
known.  The  inconceivably  great  distance  of  these 
stars  makes  them  of  peculiar  interest  and  impor- 
tance in  the  study  of  the  magnitude  and  structure 
of  the  sidereal  system. 


84 


FIG.  60. — Model  of  loo-inch  Reflector  Mounting. 

But  in  addition  to  its  power  of  reaching  out  to 
such  great  distances,  a  loo-inch  telescope  should 
also  contribute  in  large  degree  to  the  solution  of 
other  questions.  It  would  collect  more  than  twice 
as  much  light  as  the  6o-inch  reflector,  and  for  all 
classes  of  work  this  would  mean  a  great  gain. 


85 

For  example,  the  number  of  stars  whose  spectra 
could  be  photographed  on  a  sufficient  scale  to 
determine  their  radial  motions  would  be  trebled. 
At  present,  the  number  of  these  objects  is  so  re- 
stricted that  conclusions  based  on  their  study  are 
confined  to  a  comparatively  small  region  in  space 
closely  surrounding  the  sun. 


FIG.  61. — Driving  Clock  for  zoo-inch  Reflector. 

Quite  as  important  would  be  the  possibility  of 
photographing  the  spectra  of  the  brighter  stars 
with  high  dispersion.  For  determinations  of  stel- 
lar motions  the  spectrographs  now  in  use  are  very 
efficient.  But  in  many  other  respects  our  present 
position  in  stellar  spectroscopy  is  closely  analogous 


86 

to  that  which  existed  in  the  case  of  sun-spots 
before  high  dispersion  had  been  applied  to  the 
analysis  of  their  light.  Many  stellar,  lines  which 
appear  single  under  low  dispersion  are  actually 
"blends"  of  several  lines,  and  small  displacements 
due  to  pressure  and  other  physical  phenomena  in 
stellar  atmospheres  are  beyond  the  reach  of  exist- 
ing instruments.  As  already  stated,  a  beginning 
has  been  made  in  attacking  such  problems  with  the 
6o-inch  reflector,  but  the  greater  light-collecting 
power  of  the  loo-inch  telescope  is  required  for  the 
continuation  of  this  investigation. 

In  view  of  certain  statements  in  the  public  press, 
it  may  be  remarked  here  that  faint  stars  photo- 
graphed for  the  first  time  with  large  telescopes  are 
not  regarded  by  astronomers  as  discoveries,  just 
as  the  first  detection  of  a  new  sun-spot  is  not 
considered  in  this  light.  But  the  value  of  a  large 
telescope  does  come  in  part  from  its  power  of 
bringing  such  faint  stars  within  the  range  of  inves- 
tigation, as  their  study  may  add  largely  to  our 
knowledge  of  the  universe. 

The  advantages  of  great  light-gathering  power 
were  appreciated  by  the  late  Mr.  John  D.  Hooker, 
of  Los  Angeles,  wrho  in  1906  gave  to  the  Mount 
Wilson  Observatory  a  sum  of  money  to  provide  a 
telescope  mirror  of  100  inches  aperture.  Many 
difficulties  have  been  experienced  in  obtaining  a 
suitable  disk  of  glass,  on  account  of  the  great 
thickness  (13  inches)  required  to  prevent  bending. 
A  disk  weighing  \\  tons,  made  by  the  St.  Gobain 


8? 

Plate  Glass  Company  of  Paris,  has  been  thoroughly 
tested  by  the  most  exacting  methods  and  is  now 
being  polished  and  figured  in  our  optical  shop. 

To  carry  such  a  mirror  with  perfect  freedom 
from  flexure  and  with  the  high  precision  which 
modern  photographic  methods  demand,  a  tele- 
scope mounting  of  exceptional  size  is  required. 


FIG.  62. — Erecting  Steel  Building  for  loo-inch  Reflector. 

This  is  now  under  construction  at  the  Fore  River 
Ship  Yard  in  Quincy,  Massachusetts,  where  ma- 
chinery used  for  building  the  largest  battleships 
is  available.  The  smaller  parts  of  the  mounting, 
and  others  requiring  special  accuracy  of  fitting, 
are  being  made  in  our  own  machine-shop  in 
Pasadena. 


88 


The  concrete  pier  which  is  to  support  the  100- 
inch  telescope  has  already  been  built  on  Mount 
Wilson,  and  the  steel  building,  which  is  to  be  sur- 
mounted by  a  revolving  dome  100  feet  in  diameter, 
is  now  being  erected.  It  is  not  expected  that  the 
telescope  will  be  ready  for  use  before  1916. 


FIG.  63. — Dome  for  loo-inch  Reflector  in  process  of  erection, 
June,  1915. 


WORK  OF    THE  FUTURE. 

In  closing  this  brief  survey,  which  attempts  no 
more  than  to  give  an  idea  of  the  Observatory's 
activities  through  certain  typical  examples,  a  word 
may  be  said  regarding  the  future.  No  predictions 
can  be  made  as  to  possible  new  results,  as  the 
capacities  of  the  loo-inch  reflector  are  still  untried 


and  the  course  of  research  in  any  field  can  never 
be  clearly  foreseen.  In  the  study  of  the  sun  there 
is  much  to  be  done  in  continuation  and  extension 
of  present  work:  the  development,  on  an  observa- 
tional and  experimental  basis,  of  a  theory  of  sun- 
spots;  the  determination  of  the  exact  position  of 
the  sun's  magnetic  poles  and  their  period  of  revo- 


FIG.  64. — Snow  on  Mount  Wilson. 

lution  about  the  poles  of  rotation;  the  extended 
investigation  of  the  electric  and  magnetic  phe- 
nomena, and  the  pressures  and  motions  of  gases 
at  different  levels  in  the  solar  atmosphere.  Stellar 
problems  are  innumerable,  but  the  work  already 
begun  demands  first  consideration.  We  must 
learn  beyond  doubt  whether  light  is  scattered  in 


90 

space,  and  measure  the  amount  of  scattering  for 
unit  distance  if  a  positive  conclusion  is  reached. 
This  once  accomplished,  the  determination  of  a 
star's  redness,  after  due  consideration  of  the  scat- 
tering and  absorbing  effect  -of  its  own  atmosphere, 
will  afford  a  measure  of  its  distance,  thus  pro- 
viding an  invaluable  means  of  sounding  the  great- 
est depths  of  the  universe.  The  closely  entangled 
question  of  the  dependence  of  a  star's  redness  upon 
its  physical  condition,  so  important  from  the  evo- 
lutional standpoint,  must  also  be  cleared  up. 
Another  capital  problem  is  the  actual  extent  of 
Kapteyn's  two  star-streams,  which  the  increased 
light-gathering  power  of  the  loo-inch  reflector 
should  help  to  reveal.  Then  there  are  such  fun- 
damental phenomena  as  the  observed  increase  of 
a  star's  speed  with  its  age;  the  peculiar  character- 
istics of  globular  star-clusters,  which  contain  so 
many  variable  stars  of  short  period;  the  significant 
and  intricate  changes  of  variable  stars  of  all  classes; 
the  constitution  and  distribution  of  the  nebulae. 
But  many  pages  would  be  needed  to  enumerate  the 
problems  which  press  for  solution,  and  from  which 
a  selection  must  be  made. 

In  the  laboratory  the  opportunities  for  useful 
work  are  hardly  less  numerous.  Tempted  as  we 
often  are  toward  the  study  of  purely  physical 
questions,  our  chief  effort  must  be  devoted  to  the 
interpretation  of  astronomical  phenomena.  Much 
attention  will  be  given  to  the  investigation  of  the 
Zeeman  effect  and  the  phenomena  of  radiation  at 


different  temperatures  and  pressures.  The  recent 
discovery  of  the  Stark  effect  affords  an  opportunity 
to  determine  the  influence  of  electric  fields  on  light, 
and  to  apply  the  results  to  the  solar  atmosphere. 
And  as  the  prolific  researches  of  modern  physics 
continue  to  develop,  much  new  assistance  may  be 
expected  for  astrophysical  inquiries. 


FIG.  65. — Lights  of  Pasadena  and  Los  Angeles  from  Mount  Wilson. 

The  outlook  is  thus  a  favorable  one,  from  what- 
ever standpoint  it  is  viewed.  The  reappearance 
of  numerous  sun-spots,  after  a  long  period  of  solar 
calm,  and  the  approaching  completion  of  the 
loo-inch  reflector,  should  encourage  research  and 
stimulate  progress  in  all  departments  of  the 
Observatory's  work. 


PUBLISHED  PAPERS  OF  THE  OBSERVATORY,  SHOW- 
ING THE  AUTHORSHIP  OF  THE  VARIOUS  RE- 
SEARCHES HITHERTO  CONDUCTED. 

CONTRIBUTIONS  FROM  THE  MOUNT  WILSON 
SOLAR  OBSERVATORY. 

Reprinted  from  the  Astrophysical  Journal. 
VOLUME  i. 

1.  A  Study  of  the  Conditions  for  Solar  Research  at  Mount 

Wilson,  California.     George  E.  Hale.     27  pp. 

2.  The   Solar   Observatory   of   the   Carnegie    Institution   of 

Washington.     George  E.  Hale.     22  pp.,  5  pi. 

3.  A  Program  of  Solar  Research.     George  E.  Hale.     5  pp. 

4.  Some  Tests  of  the  Snow  Telescope.     George  E.  Hale.    5  pp., 

3P1- 

5.  Photographic  Observations  of  the  Spectra  of  Sun-Spots. 

George  E.  Hale  and  Walter  S.  Adams.    34  pp.,  2  pi. 

6.  Some  Notes  on  the  H  and  K  Lines  and  the  Motion  of  the 

Calcium  Vapor  in  the  Sun.     Walter  S.  Adams.     9  pp. 

7.  The  Five-Foot  Spectroheliograph  of  the  Solar  Observatory. 

George  E.  Hale  and  Ferdinand  Ellerman.     10  pp.,  3  pi. 

8.  Sun-Spot  Lines  in  the  Spectra  of  Red  Stars.     George  E. 

Hale  and  Walter  S.  Adams.     6  pp. 

9.  Latitude  and  Longitude  of  the  Solar  Observatory.    George 

E.  Hale.     4  pp. 

10.  The  Spectroscopic  Laboratory  of  the  Solar  Observatory. 

George  E.  Hale.     7  pp.,  i  pi. 

11.  Preliminary  Paper  on  the  Cause  of  the  Characteristic  Phe- 

nomena of  Sun-Spot  Spectra.     George  E.  Hale,  Walter 
S.  Adams,  and  Henry  G.  Gale.     29  pp. 

12.  Sun-Spot  Lines  in  the  Spectrum  of  Arcturus.     Walter  S. 

Adams.     9  pp. 

13.  A  loo-Inch  Mirror  for  the  Solar  Observatory.     George  E. 

Hale.     5  pp. 

14.  A  Vertical  Ccelostat  Telescope.     George  E.  Hale.     6  pp., 

i  pi. 

15.  Second  Paper  on  the  Cause  of  the  Characteristic  Phenom- 

ena of  Sun-Spot  Spectra.     George  E.  Hale  and  Walter 
S.  Adams.     20  pp.,  2  pi. 

16.  The  Heliomicrometer.     George  E.  Hale.     7  pp.,  i  pi. 

(92) 


93 

1 7-  A  Photographic  Comparison  of  the  Spectra  of  the  Limb 
and  Center  of  the  Sun.  George  E.  Hale  and  Walter  S. 
Adams,  n  pp.,  3  pi. 

1 8.  Some  New  Applications  of  the  Spectroheliograph.     George 

E.  Hale.     4  pp.,  i  pi. 

19.  The  Absence  of  Very  Long  Waves  from  the  Sun's  Spectrum. 

E.  F.  Nichols.     3  pp. 

20.  Spectroscopic  Observations  of  the  Rotation  of  the  Sun 

Walter  S.  Adams.     22  pp. 

2 1 .  Sun-Spot  Bands  which  Appear  in  the  Spectrum  of  a  Calcium 

Arc  Burning  in  the  Presence  of  Hydrogen.    Charles  M 
Olmsted.     4  pp.,  i  pi. 

22.  Preliminary   Catalogue  of  Lines  Affected   in  Sun-Spots. 

Region  X  4000  to  X  4500.     Walter  S.  Adams.     21  pp 

23.  The  Tower  Telescope  of  the  Mount  Wilson  Solar  Observa- 

tory.    George  E.  Hale.     9  pp.,  3  pi. 

24.  Preliminary  Note  on  the  Rotation  of  the  Sun  as  Determined 

from    the    Displacements    of    the    Hydrogen    Lines. 
Walter  S.  Adams.     6  pp. 

25.  Preliminary  Note  on  the  Rotation  of  the  Sun  as  Deter- 

mined from  the  Motions  of  the  Hydrogen  Flocculi. 
George  E.  Hale,     u  pp.,  3  pi. 

26.  Solar  Vortices.     George  E.  Hale.     17  pp.,  n  pi. 

27.  The   Pasadena  Laboratory  of   the   Mount  Wilson  Solar 

Observatory.     George  E.  Hale.     6  pp.,  2  pi. 

28.  An  Electric  Furnace  for  Spectroscopic  Investigations,  with 

Results  for  the  Spectra  of  Titanium  and  Vanadium. 
Arthur  S.  King.     15  pp.,  3  pi. 

29.  Anomalous  Refraction  Phenomena  Investigated  with  the 

Spectroheliograph.     W.  H.  Julius,     n  pp.,  i  pi. 

30.  On  the  Probable  Existence  of  a  Magnetic  Field  in  Sun- 

Spots.     George  E.  Hale.     29  pp.,  4  pi. 

VOLUME  2. 

31.  On  the  Absorption  of  Light  in  Space.  J.  C.Kapteyn.  9  pp. 

32.  The  Relative  Intensities  of  the  Calcium  Lines  H,  K,  and 

X4227   in    the    Electric  Furnace.     Arthur   S.   King. 
8  pp.,  i  pi. 

33.  Spectroscopic  Investigations  of  the  Rotation  of  the  Sun 

during  the  Year  1908.     Walter  S.  Adams.     36  pp. 

34.  On  the  Separation  in  the  Magnetic  Field  of  Some  Lines 

Occurring  as  Doublets,  and  Triplets  in  Sun-Spot  Spec- 
tra.    Arthur  S.  King.     8  pp.,  i  pi. 

35.  The  Relative  Intensities  of  the  Yellow,  Orange,  and  Red 

Lines  of  Calcium  in  Electric  Furnace  Spectra.    Arthur 
S.  King.     8  pp.,  i  pi. 


94 

36.  The  6o-Inch  Reflector  of  the  Mount  Wilson  Solar  Observa- 

tory.    G.  W.  Ritchey.     12  pp.,  4  pi. 

37.  Note  on  the  Polarizing  Effect  of  Coelostat  Mirrors.   Charles 

E.  St.  John.     4  pp. 

38.  A  Further  Study  of  the  H  and  K  Lines  of  Calcium.    Arthur 

S.  King.     9  pp.,  2  pi. 

39.  The  Zeeman  Effect  for  Titanium.     Arthur  S.  King.     13 

pp.,  2  pi. 

40.  A  Summary  of  the  Results  of  a  Study  of  the  Mount  Wilson 

Photographs  of  Sun-Spot  Spectra.     Walter  S.  Adams. 
41  pp. 

41.  Photography  of  the  "Flash"  Spectrum  without  an  Eclipse. 

George  E.  Hale  and  Water  S.  Adams.     9  pp.,  i  pi. 

42.  On  the  Absorption  of  Light  in  Space.     Second  Paper.    J. 

C.  Kapteyn.     36  pp. 

43.  An  Investigation  of  the  Displacements  of  the  Spectrum 

Lines  at  the  Sun's  Limb.     Walter  S.  Adams.     32  pp., 
i  pi. 

44.  The  Absolute  Wave-Lengths  of  the  H  and   K  Lines  of 

Calcium  in  Some  Terrestrial  Sources.     Charles  E.  St. 
John.     14  pp. 

45.  On  Certain  Statistical  Data  Which  May  be  Valuable  in  the 

Classification  of  the  Stars  in  the  Order  of  their  Evolu- 
tion.    J.  C.  Kapteyn.     12  pp. 

46.  The  Correspondence  between  Zeeman  Effect  and  Pressure 

Displacement  for  the  Spectra  of  Iron,  Chromium,  and 
Titanium.     Arthur  S.  King.     26  pp. 

47.  On  Some  Methods  and  Results  in  Direct  Photography  with 

the  6o-Inch  Reflecting  Telescope  of  the  Mount  Wilson 
Solar  Observatory.     G.  W.  Ritchey.     10  pp.,  6  pi. 

48.  The  General  Circulation  of  the  Mean-  and   High-Level 

Calcium  Vapor  in  the  Solar  Atmosphere.     Charles  E. 
St.  John.     47  pp.,  3  pi. 

49.  The  Spectra  of  Spiral  Nebulae  and  Globular  Star  Clusters. 

E.  A.  Path.     6  pp. 

50.  Some  Results  of  a  Study  of  the  Spectra  of  Sirius,  Procyon, 

and  Arcturus  with  High  Dispersion.  Walter  S.  Adams. 
8  pp. 

51.  Photographic  Observations  of  Prominences.    Giorgio  Abetti 

and  Ruth  Emily  Smith.     20  pp. 

52.  The  Zeeman  Effect  for  Chromium.     Harold  D.  Babcock. 

17  pp.,  2  pi. 


95 

VOLUME  3. 

53.  The  Effect  of  Pressure  upon  Electric  Furnace  Spectra. 

Arthur  S.  King.     20  pp.,  2  pi. 

54.  Motion  and  Condition  of  Calcium  Vapor  over  Sun-Spots 

and  other  Special  Regions.     Charles  E.  St.  John.     45 
pp.,  2  pi. 

55.  The  Zeeman  Effect  for  Vanadium.     Harold  D.  Babcock. 

16  pp. 

56.  The  Influence  of  a  Magnetic  Field  upon  the  Spark  Spectra 

of  Iron  and  Titanium — Summary  of  Results.    Arthur 
S.  King.     26  pp.,  4  pi. 

57.  Note  on  the  Grouping  of  Triplet  Separations  Produced  by 

a  Magnetic  Field.     Harold  D.  Babcock.     6  pp. 

58.  An  Investigation  of  the  Spectra  of  Iron  and  Titanium  under 

Moderate  Pressures.     Henry  G.  Gale  and  Walter  S. 
Adams.     38  pp.,  3  pi. 

59.  The  Three-Prism  Stellar  Spectrograph  of  the  Mount  Wilson 

Solar  Observatory.     Walter  S.  Adams.     20  pp.,  i  pi. 

60.  The  Effect  of  Pressure  upon  Electric  Furnace  Spectra. 

Second  Paper.     Arthur  S.  King.     30  pp.,  3  pi. 

61.  Tertiary  Standards  with  the  Plane  Grating.    The  Testing 

and  Selection  of  Standards.    First  Paper.    Charles  E. 
St.  John  and  L.  W.  Ware.     40  pp.,  i  pi. 

62.  Observations  of  the  Spectrum  of  Nova  Geminorum  No.  2. 

Walter  S.Adams  and  Arnold  Kohlschutter.    29  pp.,  3  pi. 

63.  The  Integrated  Spectrum  of  the  Milky  Way.     E.  A.  Path. 

6  pp.,  i  pi. 

64.  The  Algol  Variable  RR  Draconis.     Frederick  H.  Seares. 

17  pp. 

65.  On  the  Occurrence  of  the  Enhanced  Lines  of  Titanium  in 

Electric  Furnace  Spectra.     Arthur  S.  King,     n  pp., 

3P1- 

66.  The  Variation  with  Temperature  of  the  Electric  Furnace 

Spectrum  of  Iron.     Arthur  S.  King.     43  pp. 

67.  The  Spectra  of  Spiral  Nebulae  and  Globular  Star  Clusters. 

Third  Paper.     E.  A.  Path.     6  pp. 

68.  The  Algol  Variable  RR  Draconis.     Second  Paper.    Fred- 

erick H.  Seares.     18  pp. 

69.  Radial    Motion    in    Sun-Spots.     I.  The    Distribution    of 

Velocities  in  the  the  Solar  Vortex.     Charles  E.   St. 
John.     32  pp.,  I  pi. 


96 

.  VOLUME  4. 

70.  The  Photographic  Magnitude  Scale  of  the  North  Polar 

Sequence.     Frederick  H.  Scares.     27  pp. 

71.  Preliminary  Results  of  an  Attempt  to  Detect  the  General 

Magnetic  Field  of  the  Sun.     George  E.  Hale.     74  pp., 

4  pl- 

72.  The  Displacement-Curve  of  the  Sun's  General  Magnetic 

Field.     Frederick  H.  Scares.     27  pp. 

73.  A  Study  of    the    Relation  of   Arc    and  Spark  Lines  by 

Means  of  the  Tube- Arc.     Arthur  S.  King.    26  pp.,  2  pi. 

74.  Radial  Motion  in  Sun-Spots.     II.  The  Distribution  of  the 

Elements  in  the  Solar  Atmosphere.  Charles  E.  St. 
John.  51  pp. 

75.  Tertiary  Standards  with  the  Plane  Grating.    The  Testing 

and  Selection  of  Standards.  Second  Paper.  Charles 
E.  St.  John  and  L.  W.  Ware.  24  pp. 

76.  The  Variation  with  Temperature  of  the  Electric  Furnace 

Spectrum  of  Titanium.     Arthur  S.  King.     27  pp.,  i  pi. 

77.  An  Application  of  the  Registering  Micro-Photometer  to 

the  Study  of  Certain  Types  of  Laboratory  Spectra. 
Arthur  S.  King  and  Peter  Paul  Koch.  1 7  pp. 

78.  Note  on  the  Relative  Intensity  at  Different  Wave-Lengths 

of  the  Spectra  of  Some  Stars  having  Large  and  Small 
Proper  Motions.  Walter  S.  Adams.  4  pp.,  i  pi. 

79.  The  Radial  Velocities  of  One  Hundred  Stars  with  Measured 

Parallaxes.  Walter  S.  A  dams  and  A  mold  Kohlschiitter . 
9  PP- 

80.  Photographic  Photometry  with  the  6o-lnch  Reflector  of 

the  Mount  Wilson  Solar  Observatory.  Frederick  H. 
Scares.  34  pp. 

81.  The  Color  of  the  Faint  Stars.     Frederick  H.  Scares.    9   pp. 

82.  On   the   Individual   Parallaxes   of   the   Brighter   Galactic 

Helium  Stars  in  the  Southern  Hemisphere,  together 
with  Considerations  on  the  Parallax  of  Stars  in  Gen- 
eral. J.  C.  Kapteyn.  86  pp.,  i  pi. 

BEGINNING  VOLUME  5. 

83.  On  the  Change  of  Spectrum  and  Color  Index  with  Distance 

and  Absolute  Brightness.  Present  State  of  the  Ques- 
tion. J.  C.  Kapteyn.  18  pp. 

84.  A  Vertical  Adaptation  of  the  Rowland  Mounting  for  a  Con- 

cave Grating.     Arthur  S.  King.     8  pp.,  i  pi. 


97 

85.  Some  Electric  Furnace  Experiments  on  the  Emission  of 

Enhanced  Lines  in  a  Hydrogen  Atmosphere.     Arthur 
S.  King.     6  pp. 

86.  Intermediate  Degrees  of  Darkening  at  the  Limb  of  Stellar 

Disks  with  an  Application   to   the   Orbit   of  Algol. 
Harlow  Shapley.     7  pp. 

87.  The  Spectra  of  Four  of  the  Temporary  Stars.     Walter 

S.  Adams  and  F.  G.  Pease.     4  pp.,  i  pi. 

88.  On  the  Distribution  of  the  Elements  in  the  Solar  Atmos- 

phere as  given  by  Flash  Spectra.     Charles  E.  St.  John. 
21  pp. 

89.  Some  Spectral  Criteria  for  the  Determination  of  Absolute 

Stellar   Magnitudes.     Walter   S.  Adams   and  Arnold 
Kohls chiitter.     14  pp. 

90.  The  Spectroscopic  Orbit  of  RX  Herculis  Determined  from 

Three  Plates  with  a  New  Photometric  Orbit  and  Abso- 
lute Dimensions.     Harlow  Shapley.     18  pp. 

91.  New    Variables  in  the    Center    of  Messier    3.      Harlow 

Shapley.     5  pp. 

92.  On  the  Nature  and  Cause  of  Cepheid  Variation.    Harlow 

Shapley.     18  pp. 

93.  Anomalous  Dispersion  in  the  Sun  in  the  Light  of  Obser- 

vations.    Charles  R.  St.  John.     44  pp. 

94.  The  Variation  with  Temperature  of  the  Electric  Furnace 

Spectra  of  Vanadium  and  Chromium.    Arthur  S.  King. 
30  pp.,  3  pi. 

95.  The  Flash  Spectrum  without  an  Eclipse,  Region  \48oo- 

\66oo.     Walter  S.  Adams  and  Cora  G.  Burwell.    31  pp. 

96.  List  of  Stars  with  Proper  Motion  Exceeding  o''5O  Annu- 

ally.    Adriaan  van  Maanen.     In  press. 

97.  Photographic  and  Photovisual  Magnitudes  of  Stars  near 

the  North  Pole.     Frederick  H.  Seares.     In  press. 

98.  A  Comparison  of  the  Harvard  and  Mount  Wilson  Scales 

of    Photographic    Magnitude.     Frederick    H.    Seares. 
In  press. 

99.  Miscellaneous  Notes  on  Variable  Stars.     Harlow  Shapley. 

In  press. 

100.  Effective   Wave-Lengths    of    184    Stars   in    the    Cluster 

N.G.C.  1647.     Ejnar  Hertzsprung.     In  press. 

101.  Effective    Wave-Lengths    of    Absolutely    Faint    Stars. 

Ejnar  Hertzsprung.     In  press. 

102.  Color  Indices  in  the  Cluster  N.G.  C.  1647.     Frederick  H. 

Seares.     In  press. 

103.  Researches  on  Solar  Vortices.     Carl  Stormer.     In  press. 


98 

COMMUNICATIONS  TO  THE  NATIONAL  ACADEMY  OF  SCIENCES. 

Reprinted  from  the  Proceedings  of  the  Academy. 

BEGINNING  VOLUME  I. 

1.  The  Relations  Between  the  Proper  Motions  and  the  Radial 

Velocities  of  the  Stars  of  the  Spectral  Types  F,  G,  K, 
and  M.     J.  C.  Kapteyn  and  Walter  S.  Adams. 

2.  Critique  of  the  Hypothesis  of  Anomalous  Dispersion  in 

Certain  Solar  Phenomena.     Charles  E.  St.  John. 

3.  An  Attempt  to  Measure  the  Free  Electricity  in  the  Sun's 

Atmosphere.     George  E.  Hale  and  Harold  D.  Babcock. 

4.  Results  of  an  Investigation  of  the  Flash  Spectrum  Without 

an  Eclipse,  Region  \48oo  to  X66oo.     Walter  S.  Adams 
and  Cora  G.  Burwell. 

5.  Variability  of  Spectrum  Lines  in  the  Iron  Arc.     Charles 

E.  St.  John  and  Harold  D.  Babcock. 

6.  Photographic  Determination  of  Stellar  Parallaxes  with  the 

6o-inch  Reflector.     Adriaan  van  Maanen. 

7.  On  the  Pole  Effect  in  the  Iron  Arc.     Charles  E.  St.  John 

and  Harold  D.  Babcock. 

8.  Absolute  Scales  of  Photographic  and  Photo  visual  Magni- 

tude.    Frederick  H.  Seares. 

9.  Unsymmetrical  Lines  in    Tube-Arc    and    Spark  Spectra 

as  an  Evidence  of  Displacing  Action  in  these  Sources. 
Arthur  S.  King. 

10.  The  Direction  of  Rotation  of  Sun-Spot  Vortices.     George  E. 

Hale. 

1 1 .  Some  Vortex  Experiments  bearing  on  the  Nature  of  Sun- 

Spots  and  Flocculi.     George  E.   Hale  and  George   P. 
Luckey. 

12.  Nova  Geminorum  No.  2  as  a  Wolf-Rayet  Star.      Walter 

S.  Adams  and  Francis  G.  Pease. 

13.  The  Radial  Velocities  of  the  more  Distant  Stars.    Walter 

S.  Adams. 

PAPERS  FROM  THE  MOUNT  WILSON  SOLAR  OBSERVATORY. 

V.  i,  Pt.  i,  Carnegie  Institution  Publication  No.  138.  An  In- 
vestigation of  the  Rotation  Period  of  the  Sun  by  Spec- 
troscopic  Methods.  Walter  S.  A  dams  and  J.  B.  Lasby. 
132  pp.,  2  pi. 

V.  2,  Pt.  i,  Carnegie  Institution  Publication  No.  153.  The 
Influence  of  a  Magnetic  Field  upon  the  Spark 
Spectra  of  Iron  and  Titanium.  Arthur  S.  King. 
66  pp.,  6  pi. 


ORGANIZATION  OF  THE  OBSERVATORY. 

GEORGE  E.  HALE,  Director. 

WALTER  S.  ADAMS,  Assistant  Director. 


RESEARCH  DIVISION. 

Solar  Physics:  George  E.  Hale,  in  charge;  Charles  E.  St.  John; 
Ferdinand  Ellerman;  Adriaan  van  Maanen;  Walter  Colby; 
George  S.  Monk;  George  P.  Luckey. 

Stellar  Spectroscopy:  Walter  S.  Adams,  in  charge;  Francis  G. 
Pease;  George  S.  Monk;  George  P.  Luckey. 

Stellar  Photometry:  Frederick  H.  Scares,  in  charge;  Harlow 
Shapley. 

Nebular  Photography:   G.  W.  Ritchey;  Francis  G.  Pease. 

Stellar  Parallax:  Adriaan  van  Maanen. 

Physical  Laboratory:  Arthur  S.  King,  in  charge;  Harold  D. 
Babcock;  Walter  Colby. 

RESEARCH  ASSOCIATES. 

J.  C.  Kapteyn,  University  of  Groningen;  Carl  Stormer,  Uni- 
versity of  Christiania. 

COMPUTING  DIVISION. 

Frederick  H.  Scares,  superintendent;  Ada  M.  Brayton;  Cora 
G.  Burwell;  Helen  Davis;  Dorothy  Bach  Fretter;  Mary  C. 
Joyner;  Merl  McClees;  Addie  L.  Miller,  Ardis  Thomas 
Monk;  Myrtle  L.  Richmond;  Bertha  M.  Shumway; 
Louise  Ware;  Laura  West;  Coral  Wolfe;  Jessie  Haines 
Longacre,  librarian. 

OFFICE  AND  DESIGN. 

Francis  G.  Pease,  instrument  design;  E.  C.  Nichols,  H.  S. 
Kinney,  William  Hookway,  draftsmen;  Deforest  S.  Mul- 
vin,  bookkeeper;  Margaret  M.  Van  Petten,  stenographer; 
Lucena  McBride,  telephone  operator. 

INSTRUMENT  CONSTRUCTION. 

Optical  Shop:  G.  W.  Ritchey,  chief  optician;  W.  L.  Kinney; 
James  Dalton. 

Instrument  Shop:  Clement  Jacomini,  chief  instrument  maker; 
Alden  F.  Ayers,  foreman;  C.  D.  Shumway;  James  Chap- 
man; Elmer  Prall;  Warren  Howell;  L.  R.  Hitchcock; 
Gardner  Sherbourne;  Ernest  Keil;  Caleb  Moore;  Albert 
Mclntire;  Max  L.  Derr;  J.  A.  Falk;  H.  B.  Terbeck; 
M.  C.  Hurlbut;  Fred  Scherff;  Oscar  Swanson;  Fred  Hen- 
sen;  J.  T.  Wagner. 

OPERATION  AND  ERECTION. 

George  D.  Jones,  superintendent  of  transportation  and  erec- 
tion; Merritt  C.  Dowd,  engineer;  Sam  Jones,  assistant 
engineer;  W.  P.  Hoge,  night  assistant;  D.  C.  Johnson, 
steward;  J.  Fred  Burt,  R.  R.  Campbell,  Joseph  Pring, 
janitors. 


14  DAY  USE 

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